[mips] Fix c.<cc>.<fmt> instruction definition.

As part of this effort, remove MipsFCmp nodes and use tablegen
patterns rather than custom lowering through C++.

Unexpectedly, this improves codesize for microMIPS as previous floating
point setcc expansions would materialize 0 and 1 into GPRs before using
the relevant mov[tf].[sd] instruction. Now $zero is used directly.

Reviewers: dsanders, vkalintiris, zoran.jovanovic

Differential Review: https://reviews.llvm.org/D23118

llvm-svn: 281022