[RISCV][NFC] Add tests for checking isnan patterns

Summary:
I worked on adding some SelectionDag patterns to address code generated by these
examples, which came out of some differential testing against GCC. The pattern
additions will be in a follow-up patch.

Reviewers: luismarques, asb

Reviewed By: luismarques, asb

Differential Revision: https://reviews.llvm.org/D78907