[SystemZ::TTI] Improve cost handling of uint/sint to fp conversions.

Let i8/i16 uint/sint to fp conversions cost 1 if operand is a load.

Since the load already does the extension, there is no extra cost (previously
returned 2).

Review: Ulrich Weigand
https://reviews.llvm.org/D54028

llvm-svn: 346009