If two call instructions were generated right after each other, the
source position table could get populated with two entries for the same
PC (triggered by the follow-up CL: https://crrev.com/c/2697359).
This CL fixes that by slightly changing the carry-over of source
positions from nodes to instructions.

The call node which has a source position attached generates two
instructions:
      18: gap () ([rax|R|tp] = v16(-); [rbx|R|t] = v17(-);)
          [rax|R|t] = ArchCallWasmFunction [immediate:4] #-1 [rax|R|tp] [rbx|R|t] [immediate:5]
      19: gap () ()
          ArchJmp [immediate:6]

Those are then reversed, and the source position is attached to the first
one (the ArchJmp). After reversing it again later, the source position
will be set to the pc *after* the call instruction, which in the example
happened to be just another call instruction which already had a source
position, resulting in this code:

[...]
0x388ee467d426    66  e875feffff     call 0x388ee467d2a0     ;; wasm stub: WasmThrow
0x388ee467d42b    6b  e850feffff     call 0x388ee467d280     ;; wasm stub: WasmStackGuard
[...]
Source positions:
 pc offset  position
        6b         5
        6b         0

By attaching the source position to the *last* instruction (after
reversing), we ensure that it will be generated for an instruction
*before* the call, or the call itself if this is the first instruction
emitted for that node.

R=jgruber@chromium.org

Bug: v8:11490, v8:11496
Change-Id: Ie95c87d0d9daea56ca14a811abcd02ac07a4cf84
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2697358
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72951}