[RISCV] Add test cases showing failure to use ADDIW before fcvt.s.w/fcvt.d.w/fcvt.h.w. NFC

By not using ADDIW we can cause both an ADDIW and ADDI to be emitted
when the add has multiple users.

These instructions needed be added to the list of instructions that
only use the lower 32 bits of input.

I've also added tests for the wu versions, but I'm having trouble
showing bad codegen from it.