PowerPC: Treat llvm.fma.f* intrinsic as using CTR with SPE

Summary:
The SPE doesn't have a 'fma' instruction, so the intrinsic becomes a
libcall.  It really should become an expansion to two instructions, but
for some reason the compiler doesn't think that's as optimal as a
branch.  Since this lowering is done after CTR is allocated for loops,
tell the optimizer that CTR may be used in this case.  This prevents a
"Invalid PPC CTR loop!" assertion in the case that a fma() function call
is used in a C/C++ file, and clang converts it into an intrinsic.

Reviewed By: shchenz
Differential Revision: https://reviews.llvm.org/D78668