Pierre van Houtryve 922fafaff8 [GlobalISel] Micro-optimize getConstantVRegValWithLookThrough (#91969) ...

I was benchmarking the MatchTable when I found that
`getConstantVRegValWithLookThrough` took a non-negligible amount of
time, about 7.5% of all of
`AArch64PreLegalizerCombinerImpl::tryCombineAll`.

I decided to take a closer look to see if I could squeeze some
performance out of it, and I landed on a few changes that:
- Avoid copying APint unnecessarily, especially returning
std::optional<APInt> can be expensive when a out parameter also works.
- Avoid indirect call by using templated function pointers instead of
function_ref/std::function

Both of those changes seem to speedup this function by about 50%, but my
benchmarking (`perf record`) seems inconsistent (so take measurements
with a grain of salt), I saw as high as 4.5% and as low as 2% for this
function on the exact same input after the changes, but it never got
close again to 7% in a few runs so this looks like a stable improvement.

2024-05-14 08:26:00 +02:00

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