Coalesce 16-bit FP types to use integer register classes.

i16/f16/bf16 will use the same .b16 registers and
i32/v2f16 and v2bf16 will share .b32 registers.

The changes are mostly mechanical, intended to remove unnecessary register
classes which tend to produce redundant register moves.

Differential Revision: https://reviews.llvm.org/D151601

v2f16 regtype conversion to i32