Improve IO read performance.

Manually extract out a separate noinline _throw_not_readable function.

Addresses
JuliaLang#28481 (comment).

Benchmark scenario:

```julia
using BenchmarkTools, Random
const N = 1000
@Btime read(buf, Float64) evals = N setup = begin
    rng = MersenneTwister(1)
    writebuf = IOBuffer()
    map(1 : N) do _
        write(writebuf, rand(rng, Float64))
    end
    buf = IOBuffer(take!(writebuf))
end
```

Benchmark results (CPU: Intel(R) Core(TM) i7-6950X CPU @ 3.00GHz):

* Before: 2.533 ns (0 allocations: 0 bytes)
* After: 1.775 ns (0 allocations: 0 bytes)

Performance is now much closer to, but still not quite at the level of
FastIOBuffers.FastReadBuffer (1.555 ns (0 allocations: 0 bytes)).

base/iobuffer.jl

@@ -155,8 +155,13 @@ show(io::IO, b::GenericIOBuffer) = print(io, "IOBuffer(data=UInt8[...], ",
                                       "ptr=",      b.ptr, ", ",
                                       "mark=",     b.mark, ")")
 
+@noinline function _throw_not_readable()
+    # See https://github.com/JuliaLang/julia/issues/29688.
+    throw(ArgumentError("read failed, IOBuffer is not readable"))
+end
+
 function unsafe_read(from::GenericIOBuffer, p::Ptr{UInt8}, nb::UInt)
-    from.readable || throw(ArgumentError("read failed, IOBuffer is not readable"))
+    from.readable || _throw_not_readable()
     avail = bytesavailable(from)
     adv = min(avail, nb)
     GC.@preserve from unsafe_copyto!(p, pointer(from.data, from.ptr), adv)
@@ -168,7 +173,7 @@ function unsafe_read(from::GenericIOBuffer, p::Ptr{UInt8}, nb::UInt)
 end
 
 function read(from::GenericIOBuffer, T::Union{Type{Int16},Type{UInt16},Type{Int32},Type{UInt32},Type{Int64},Type{UInt64},Type{Int128},Type{UInt128},Type{Float16},Type{Float32},Type{Float64}})
-    from.readable || throw(ArgumentError("read failed, IOBuffer is not readable"))
+    from.readable || _throw_not_readable()
     avail = bytesavailable(from)
     nb = sizeof(T)
     if nb > avail
@@ -184,7 +189,7 @@ end
 
 function read_sub(from::GenericIOBuffer, a::AbstractArray{T}, offs, nel) where T
     require_one_based_indexing(a)
-    from.readable || throw(ArgumentError("read failed, IOBuffer is not readable"))
+    from.readable || _throw_not_readable()
     if offs+nel-1 > length(a) || offs < 1 || nel < 0
         throw(BoundsError())
     end
@@ -200,7 +205,7 @@ function read_sub(from::GenericIOBuffer, a::AbstractArray{T}, offs, nel) where T
 end
 
 @inline function read(from::GenericIOBuffer, ::Type{UInt8})
-    from.readable || throw(ArgumentError("read failed, IOBuffer is not readable"))
+    from.readable || _throw_not_readable()
     ptr = from.ptr
     size = from.size
     if ptr > size
@@ -212,7 +217,7 @@ end
 end
 
 function peek(from::GenericIOBuffer)
-    from.readable || throw(ArgumentError("read failed, IOBuffer is not readable"))
+    from.readable || _throw_not_readable()
     if from.ptr > from.size
         throw(EOFError())
     end
@@ -506,7 +511,7 @@ end
 # copy-free crc32c of IOBuffer:
 function _crc32c(io::IOBuffer, nb::Integer, crc::UInt32=0x00000000)
     nb < 0 && throw(ArgumentError("number of bytes to checksum must be ≥ 0"))
-    io.readable || throw(ArgumentError("read failed, IOBuffer is not readable"))
+    io.readable || _throw_not_readable()
     n = min(nb, bytesavailable(io))
     n == 0 && return crc
     crc = GC.@preserve io unsafe_crc32c(pointer(io.data, io.ptr), n, crc)