[InstCombine] Simplify and correct folding fcmps with the same children

Summary: Take advantage of FCmpInst::Predicate's bit pattern and handle (fcmp *, x, y) | (fcmp *, x, y) and (fcmp *, x, y) & (fcmp *, x, y) more consistently. Also fold more FCmpInst::FCMP_FALSE and FCmpInst::FCMP_TRUE to constants.

Currently InstCombine wrongly folds (fcmp ogt, x, y) | (fcmp ord, x, y) to (fcmp ogt, x, y); this patch also fixes that.

Reviewers: spatel

Subscribers: llvm-commits, iteratee, echristo

Differential Revision: http://reviews.llvm.org/D21775

llvm-svn: 274156