[InstCombine] fold urem with sext bool divisor

Similar to other patches in this series:
https://reviews.llvm.org/rL335512
https://reviews.llvm.org/rL335527
https://reviews.llvm.org/rL335597
https://reviews.llvm.org/rL335616

...this is filling a gap in analysis that is exposed by an unrelated select-of-constants transform.
I didn't see a way to unify the sext cases because each div/rem opcode results in a different fold.

Note that in this case, the backend might want to convert the select into math:
Name: sext urem
%e = sext i1 %x to i32
%r = urem i32 %y, %e
=>
%c = icmp eq i32 %y, -1
%z = zext i1 %c to i32
%r = add i32 %z, %y

llvm-svn: 335622