Cranelift: Use bump allocation in remove_constant_phis pass

[fitzgen]

This makes compilation 2-6% faster for Sightglass's bz2 benchmark:

compilation :: cycles :: benchmarks/bz2/benchmark.wasm

  Δ = 7290648.36 ± 4245152.07 (confidence = 99%)

  bump.so is 1.02x to 1.06x faster than main.so!

  [166388177 183238542.98 214732518] bump.so
  [172836648 190529191.34 217514271] main.so

compilation :: cycles :: benchmarks/pulldown-cmark/benchmark.wasm

  No difference in performance.

  [182220055 225793551.12 277857575] bump.so
  [193212613 227784078.61 277175335] main.so

compilation :: cycles :: benchmarks/spidermonkey/benchmark.wasm

  No difference in performance.

  [3848442474 4295214144.37 4665127241] bump.so
  [3969505457 4262415290.10 4563869974] main.so

[cfallin]

This is great, thanks!

Some thoughts below for potential improvements while we're here, but feel free to defer if you'd prefer.