[OpenMP] Simplify getFloatTypeSemantics

When the float point representations are the same on the host and on the target device,
(`&Target->getLongDoubleFormat() == &AuxTarget->getLongDoubleFormat()`),
we can just use `AuxTarget->getLongDoubleFormat()`.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D64423

llvm-svn: 365545