[RISCV] Add patterns for checking isnan

Summary:
This patch addresses some weird assembly sequences we were seeing during
comparing floats. In particular, comparing a float to itself tells you whether
it is NaN or not, which we were doing correctly, but with an extra unneeded
`and` instruction.

This patch specialises the existing patterns to remove the `and` instructions
when both their operands are the same.

Reviewed By: luismarques, asb

Differential Revision: https://reviews.llvm.org/D78908