WebAssembly locals are specified to be zero on function entry. Liftoff
implements this by just storing the constant 0 in the virtual stack for
integer types, and using one floating point register initialized to
zero for all floating point types.
For big counts of locals this leads to problems (manifesting as huge
blocks of code being generated) once we hit a merge point: All those
constants (for int) and all duplicate register uses (for floats) need to
be fixed up, by using separate registers for the locals or spilling to
the stack if no more registers are available. All this spilling
generates a lot of code, and can even happen multiple times within a
function.

This CL optimizes for such cases by spilling all locals to the stack
initially. All merges within the function body get much smaller then.
The spilled values rarely have to be loaded anyway, because the initial
zero value is usually overwritten before the first use.

To optimize the code size for initializing big numbers of locals on the
stack, this CL also introduces the platform-specific
{FillStackSlotsWithZero} method which uses a loop for bigger local
counts.

This often saves dozens of kilobytes for very big functions, and shows
an overall code size reduction of 4-5 percent for big modules.

R=jkummerow@chromium.org

Bug: v8:9830
Change-Id: I23fa4145847827420f09e043a11e0e7b606e94cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1856004
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64282}