On ia32, the instruction selector uses movsx_b to compile the wasm
SignExtendWord8ToInt32 instruction. movsx_b requires a byte register
as input. However, not all allocatable registers on ia32 are. As we
cannot currently express constraints on subsets of registers, this
change now forces the input to movsx_b into eax.

Bug: chromium:919572
Change-Id: I39bd391974954ec9044940c3164398109eb78908
Reviewed-on: https://chromium-review.googlesource.com/c/1400409Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58622}