[SDAG] Handle non-integer preferred memset types for non-constant values

The existing code in getMemsetValue only handled integer-preferred types when
the fill value was not a constant. Make this more robust in two ways:

  1. If the preferred type is a floating-point value, do the mul-splat trick on
     the corresponding integer type and then bitcast.
  2. If the preferred type is a vector, do the mul-splat trick on one vector
     element, and then build a vector out of them.

Fixes PR22754 (although, we should also turn off use of vector types at -O0).

llvm-svn: 233749