[mips] Use the delay slot filler to convert branches for microMIPSR6.

[mips] Use the delay slot filler to convert branches for microMIPSR6.

The MIPS delay slot filler converts delay slot branches into compact
forms for the MIPS ISAs which support them. For branches that compare
(in)equality with with zero, it converts them into branches with implict
zero register operands. These branches have a slightly greater range
than normal two register operands branches.

Changing the branches at this point in the pipeline offers the long
branch pass the ability to mark better judgements if a long branch
sequence is required.

Reviewers: atanasyan

Differential Revision: https://reviews.llvm.org/D40314

llvm-svn: 318908