InstructionSelector::ZeroExtendsWord32ToWord64 assumes that a
Load[kRepWord8|kTypeInt32] generates a zero-extended value. This
assumption makes sense, but was not fulfilled by the instruction
selector which emitted an "ldrsb" instruction which sign-extended to the
full 64-bit register.

This CL fixes that by introducing a separate "LdrsbW" instruction which
is selected if we are sign-extending an 8-bit value to 32-bit.

R=ahaas@chromium.org, mslekova@chromium.org
CC=v8-arm-ports@googlegroups.com

Bug: chromium:1239116
Change-Id: I2da1ad6062805acf5558f3e66b8db9a50e830302
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3094011
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76283}