[SPE] Support constrained float operations on SPE

This patch enables support on SPE for constrained arithmetic and
comparison operations. This fixes bugzilla 50070.

One thing not covered is fcmp vs. fcmps on SPE. Some condition code
generates singaling comparison while some not. In this patch, all are
considered as singaling. So there might be still some issue when
compiling from C code.

Reviewed By: jhibbits

Differential Revision: https://reviews.llvm.org/D101282