Simplify GrPaint::isConstantBlendedColor to avoid FP analysis and XP virtual calls

Simplify GrPaint::isConstantBlendedColor to avoid FP analysis and XP virtual calls

This was the only non-test user of GrXPFactory::isConstantPreCoverageBlendedColor which is now removed.

Change-Id: Ic1c130d30a44e1a955b89f8912433a3c9df2e61e
Reviewed-on: https://skia-review.googlesource.com/8776
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>