[InstCombine] Use simplifyWithOpReplaced() for non-bool selects

Perform the simplifyWithOpReplaced() fold even for non-bool
selects. This subsumes a number of recently added folds for
zext/sext of the condition.

We still need to manually handle variations with both sext/zext
and not, because simplifyWithOpReplaced() only performs one
level of replacements.