[ValueTracking] Remove SPF support from `computeKnownBitsFromOperator` (#76630)

This patch removes redundant SPF support
(https://github.com/llvm/llvm-project/commit/5350e1b5096aa4707aa525baf7398d93b4a4f1a5)
from `computeKnownBitsFromOperator` as we always canonicalize a SPF into
an intrinsic call.

Compile-time improvement:
http://llvm-compile-time-tracker.com/compare.php?from=3dc0638cfc19e140daff7bf1281648daca8212fa&to=8771ef0749fb2ba4304dc68d418c88ec5769346f&stat=instructions:u

|stage1-O3|stage1-ReleaseThinLTO|stage1-ReleaseLTO-g|stage1-O0-g|stage2-O3|stage2-O0-g|stage2-clang|
|--|--|--|--|--|--|--|
-0.01%|-0.01%|+0.01%|+0.00%|+0.01%|+0.04%|-0.01%|