[InstCombine] fold mul with masked low bit operand to trunc+select

https://alive2.llvm.org/ce/z/o7rQ5q

This shows an extra instruction in some cases, but that is
caused by an existing canonicalization of trunc -> and+icmp.

Codegen should be better for any target where a multiply is
more costly than the most simple ALU op.

This ends up producing the requested x86 asm from issue #55618,
but it's not the same IR. We are missing a canonicalization
from the negate+mask pattern to the trunc+select created here.