Teach InstCombine's ComputeMaskedBits to handle pointer expressions

in addition to integer expressions. Rewrite GetOrEnforceKnownAlignment
as a ComputeMaskedBits problem, moving all of its special alignment
knowledge to ComputeMaskedBits as low-zero-bits knowledge.

Also, teach ComputeMaskedBits a few basic things about Mul and PHI
instructions.

This improves ComputeMaskedBits-based simplifications in a few cases,
but more noticeably it significantly improves instcombine's alignment
detection for loads, stores, and memory intrinsics.

llvm-svn: 49492