[ARM] Convert vector fdiv+fcvt fixed-point combine to fmul.

Instcombine will convert fdiv by a power-2 to fmul, this converts the
PerformVDIVCombine that converts fdiv+fcvt to fixed-point fcvt to fmul+fcvt.
The fdiv tests will look worse, but won't appear in practice (and should be
improved again by #93882).