[ARM] Transform a fixed-point to floating-point conversion into a VCVT_fix

[ARM] Transform a fixed-point to floating-point conversion into a VCVT_fix

Conversion from a fixed-point number to a floating-point number is done by
multiplying the fixed-point number by 2^(-n) where n is the number of
fractional bits. Currently this is lowered to a vcvt
(integer to floating-point) then a vmul, but it can instead be lowered
directly to a vcvt (fixed-point to floating-point). This patch enables
such transformations as long as the multiplication factor is a power of 2.

Differential Revision: https://reviews.llvm.org/D103903