[SystemZ] Use load-and-test for fp compare with 0 if vector support is present.

 [SystemZ] Use load-and-test for fp compare with 0 if vector support is present.

Since the LTxBRCompare instructions can't be used with vector registers, a
normal load-and-test instruction (with a modelled def operand) is used instead.

Reviewed by Ulrich Weigand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249664 91177308-0d34-0410-b5e6-96231b3b80d8