[RISCV] Print Zfa fli instruction FP values in a more adaptive way.

[RISCV] Print Zfa fli instruction FP values in a more adaptive way.

Previously, we printed all constants in scientific notation with
6 digits of precision. This is not enough to accurately display
the smallest value, but increasing the precision would be too much
for other values.

This patch prints values with fractional bits using only as many digits as
needed. 1*2^-15 and 1*2^-16 will be printed in scientific notation while
the others are printed without scientific notation. The integer values
are printed with a single 0 after the decimal point.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D145645