[RISCV] Add support for RVV int<->fp & fp<->fp conversions

This patch adds support for the full range of vector int-to-float,
float-to-int, and float-to-float conversions on legal types.

Many conversions are supported natively in RVV so are lowered with
patterns. These include conversions between (element) types of the same
size, and those that are half/double the size of the input. When
conversions take place between types that are less than half or more
than double the size we must lower them using sequences of instructions
which go via intermediate types.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D95447