[RISCV] Merge getLoadFP*Imm into a single function.

We currently have 3 functions and 3 lookup tables. This was the
most expediant and obvious way to fix several bugs.

This patch uses a single function and single lookup
table. It uses APFloat::convert to convert from the half or double
to single precision. If the conversion doesn't have any errors or
lose any information we use the f32 table to finish the lookup.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D145897