[AArch64] Try to detect patterns with fdiv and fmul for [su]cvtf.

If fmul's constant operand is the reciprocal of a power of 2 (i.e 1/2^n) or
fdiv's constant operand is power of 2, we can try to match patterns with
[su]int_to_fp for [su]cvtf.

Differential Revision: https://reviews.llvm.org/D156538