[Support] improve known bits analysis for multiply by power-of-2 (1 set bit)

This can be viewed as recognizing that multiply-by-power-of-2 doesn't
have a carry into the top bit of an M-bit * N-bit number.

Enhancing canonicalization of mul -> select might also handle some of
these if we were ok with increasing instruction count with casts in
some cases.

This doesn't help https://llvm.org/PR49055 , but it's a simpler
pattern that we miss.
Note: "-sccp" already gets these examples using a constant
range analysis.

Differential Revision: https://reviews.llvm.org/D114962