[RISCV] Adjust how -1.0 is handled in RISCVLoadFPImm. NFC

Instead of hardcoding the -1.0 exponent and mantissa separately from the
table, reuse the table entry for 1.0.

When searching the table we can change index 16 to index 0 if the sign
is negative. Or when indexing we can change index 0 to index 16 and remember
that we need to flip the sign.

I'm going to make another patch that has a f16 and f64 table and
this makes the code for those more similar.