[LegalizeIntegerTypes][RISCV] Teach PromoteSetCCOperands to check sign bits of unsigned compares.

Unsigned compares work with either zero extended or sign extended
inputs just like equality comparisons. I didn't allow this when
I refactored the code in D116421 due to lack of tests. But I've
since found a simple C test case that demonstrates when this can be
useful.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D116617