[RISCV] Merge fp-imm.ll and zfh-imm.ll into float/double/half-imm.ll. NFC

fp-imm.ll and zfh-imm.ll test 0.0 and -0.0 while float/double/half-imm.ll
tested other non-zero constants. It seems like they should all be
tested together.

There are slight coverage changes due to different command lines,
but I'm not sure its meaningful. For example, we now don't test
double 0.0 and -0.0 with only the F extension.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D156929