[InstCombine] Use SimplifyQuery for computeKnownBits() (NFC)

Call computeKnownBits() with SimplifyQuery to make sure it gets
all available analyses, even if more are added in the future.

As this code is performance-critical, I'm exporting the variant
with by-ref KnownBits and SimplifyQuery, as the variant returning
KnownBits is measurably slower in this context.