This adds support to TurboFan's representation selection for the Word64
representation, and makes use of that to handle indices for memory access
and allocation instructions (i.e. LoadElement, StoreElement, Allocate,
etc.). These instructions had previously used Word32 as representation
for the indices / sizes, and then internally converted it to the correct
representation (aka Word64 on 64-bit architectures) later on, but that
was kind of brittle, and sometimes led to weird generated code.

The change thus only adds support to convert integer values in the safe
integer range from all kinds of representations to Word64 (on 64-bit
architectures). We don't yet handle the opposite direction and none of
the representation selection heuristics for the numeric operations were
changed so far. This will be done in follow-up CLs.

This CL itself is supposed to be neutral wrt. functionality, and only
serves as a starting point, and a cleanup for the (weird) implicit
Word64 index/size handling.

Bug: v8:7881, v8:8015, v8:8171
Design-Document: http://bit.ly/turbofan-word64
Change-Id: I3c6961a0e96cbc3fb8ac9d3e1be8f2e5c89bfd25
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel
Reviewed-on: https://chromium-review.googlesource.com/1224932
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55886}