[AArch64] Avoid expensive getStrictlyReservedRegs calls in isAnyArgRegReserved (#190957)
`AArch64RegisterInfo::isAnyArgRegReserved` is used during call lowering
across all instruction selectors (SDAG, GISel, FastISel) to emit an
error if any of the arg registers (x0-x7) are reserved. This puts
`AArch64RegisterInfo::getStrictlyReservedRegs` which computes this in
the hot-path and it shows up in compile-time profiles since it's
computed for every call.
As the intent was to guard against using +reserve-x{0-7} with function
calls we can instead call `isXRegisterReserved` which is faster since
it's a simple BitVector lookup.
Compile-time improves across all instruction selectors on CTMark:
geomean
SDAG ~ -0.14%
GISel ~ -0.6%
FastISel ~ -0.7% (measured locally)
https://llvm-compile-time-tracker.com/compare.php?from=4d84263b341c3ee1847081c50e1541b62fbcb08c&to=23c4576c01d7d5e0e603366b86239e069410c3a7&stat=instructions%3Au
The numbers above are from the initial implementation that cached
`isAnyArgRegReserved`, but local experiments show instead calling
`isXRegisterReserved` is even faster. I also experimented with caching
`getStrictlyReservedRegs`, but the compile-time impact is negligible which
suggests this code isn't particularly hot and worth optimising outside of
call-lowering:
https://llvm-compile-time-tracker.com/compare.php?from=4d84263b341c3ee1847081c50e1541b62fbcb08c&to=38166d5afa9dd53320da0d47d4f056d08f0d75ca&stat=instructions%3Au
so I've also removed the FIXME from `getStrictlyReservedRegs`.
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