[InstCombine] allow multi-use values in canEvaluate* if all uses are in 1 inst

This is the enhancement suggested in D42536 to fix a shortcoming in 
regular InstCombine's canEvaluate* functionality.
When we have multiple uses of a value, but they're all in one instruction, we can 
allow that expression to be narrowed or widened for the same cost as a single-use 
value.

AFAICT, this can only matter for multiply: sub/and/or/xor/select would be simplified 
away if the operands are the same value; add becomes shl; shifts with a variable shift 
amount aren't handled.

Differential Revision: https://reviews.llvm.org/D42739

llvm-svn: 324014