[SelectionDAG] Allow sin/cos -> sincos optimization on GNU triples w/ just -fno-math-errno

Summary:
This change enables the sin(x) cos(x) -> sincos(x) optimization on GNU
target triples.  This optimization was being inhibited when -ffast-math
wasn't set because sincos in GLibC does not set errno, while sin and cos
do.  However, this optimization will only run if the attributes on the
sin/cos calls include readnone, which is how clang represents the fact
that it doesn't care about the errno values set by these functions (via
the -fno-math-errno flag).

Reviewers: hfinkel, bogner

Subscribers: mcrosier, javed.absar, llvm-commits, paul.redmond

Differential Revision: https://reviews.llvm.org/D32921

llvm-svn: 305204