inhibit the sincos optimization when the target has sin and cos instructions

[Cesar Philippidis]

The cse_sincos pass tries to optimize sequences such as

  sin (x);
  cos (x);

into a single call to sincos, or cexpi, when available. However, the
nvptx target has sin and cos instructions, albeit with some loss of
precision (so it's only enabled with -ffast-math). This patch teaches
cse_sincos pass to ignore sin, cos and cexpi instructions when the
target can expand those calls. This yields a 6x speedup in 314.omriq
from spec accel when running on Nvidia accelerators.

Is this OK for trunk?

Cesar
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