[SLPVectorizer] Check that lowered type is floating point before calling isFabsFree

In the case of soft-fp (e.g. fp128 under wasm) the result of
getTypeLegalizationCost() can be an integer type even if the input is
floating point (See LegalizeTypeAction::TypeSoftenFloat).

Before calling isFabsFree() (which asserts if given a non-fp
type) we need to check that that result is fp.  This is safe since in
fabs is certainly not free in the soft-fp case.

Fixes PR39168

Differential Revision: https://reviews.llvm.org/D52899

llvm-svn: 344069