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

Much like fixed-point to floating-point conversion, the converse can
also be transformed into a fixed-point VCVT. This patch transforms
multiplications of floating point numbers by 2^n into a VCVT_fix. The
exception is that a float to fixed conversion with 1 fractional bit
ends up being an FADD (FADD(x, x) emulates FMUL(x, 2)) rather than an FMUL so there is a special case for that. This patch also moves the code from https://reviews.llvm.org/D103903 into a separate function as fixed to float and float to fixed are very similar.

Differential Revision: https://reviews.llvm.org/D104793