Allow redefining variables inside lets

[fitzgen]

This example redefines carry and hi_shifted:

(decl shl_i128 (ValueRegs Reg) ValueRegs)
(rule (shl_i128 src amt)
      ;; Unpack the registers that make up the 128-bit value being shifted.
      (let ((src_lo Reg (value_regs_get src 0))
            (src_hi Reg (value_regs_get src 1))
            ;; Do two 64-bit shifts.
            (lo_shifted Reg (shl $I64 src_lo (Imm8Reg.Reg amt)))
            (hi_shifted Reg (shl $I64 src_hi (Imm8Reg.Reg amt)))
            ;; `src_lo >> (64 - shift)` are the bits to carry over from the lo
            ;; into the hi.
            (carry Reg (shr $I64 src_lo (Imm8Reg.Reg (sub $I64 (imm $I64 64) (RegMemImm.Reg amt)))))
            (zero Reg (imm $I64 0))
            ;; Nullify the carry if we are shifting in by a multiple of 128.
            (carry Reg (with_flags_1 (test (OperandSize.Size64) (RegMemImm.Imm 127) amt)
                                     (cmove $I64 (CC.Z) (RegMem.Reg zero) carry)))
            ;; Add the carry into the high half.
            (hi_shifted Reg (or $I64 carry (RegMemImm.Reg hi_shifted))))
        ;; Combine the two shifted halves. However, if we are shifting by 64,
        ;; then the low bits are zero and the high bits are our low bits.
        (with_flags_2 (test (OperandSize.Size64) (RegMemImm.Imm 64) amt)
                      (cmove $I64 (CC.Z) (RegMem.Reg lo_shifted) zero)
                      (cmove $I64 (CC.Z) (RegMem.Reg hi_shifted) lo_shifted))))

Without the ability to redefine, we have to use names like carry2 and hi_shifted2 or something, like Erlang.

[cfallin]

👍 to this; it'd be consistent with the expected lexical scoping rules for lets from other S-expr languages.