InstructionSimplify: End our long national nightmare of ever-growing Simplify* arguments.

Summary:
Expose the internal query structure, start using it.

Note: This is the most minimal change possible i could create.  I have
trivial followups, like fixing the one use of const FastMathFlags &,
the renaming of CtxI to be consistent, etc.

This should be NFC.

Reviewers: majnemer, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32448

llvm-svn: 301379