[mips] Correct the patterns for bswap

Guard the MIPS64 variant correctly for i64, mark the MIPS32 version as not
in microMIPS and provide the microMIPS version.

Additionally, remove a related stale XFAIL'd test as bswap has its own test
case providing coverage.

Reviewers: smaksimovic, abeserminji, atanasyan

Differential Revision: https://reviews.llvm.org/D45816

llvm-svn: 330705