The operations are implemented:

- i16x8.load8x8_s
- i16x8.load8x8_u
- i32x4.load16x4_s
- i32x4.load16x4_u
- i64x2.load32x2_s
- i64x2.load32x2_u

on x64 and i32. The rest of the arch currently bail out, and will be
implemented in subsequent patches.

The liftoff-compiler.cc code looks very similar to the one for LoadMem,
the only difference is special handling of kSplat v.s. kExtend. kExtend
always loads 8 bytes, so the bounds check and tracing is different.
Compared to LoadMem there is less need for pinning, since the result is
always going to be in a SIMD/FP register, which is different from the
index/addr register.

The enum LoadTransformationKind was moved from
function-body-decoder-impl.h to function-body-decoder.h so that no
unncessary header file inclusions were needed to liftoff, and also it's
a better place for it to live.

Bug: v8:9909
Change-Id: I926bcc01c0c3c860223e8c08f91bc4ab3b75c399
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2203730
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67914}