[mips] Mark the `lwupc` instruction as MIPS64 R6 only

The "The MIPS64 Instruction Set Reference Manual" [1] states that
the `lwupc` is MIPS64 Release 6 only. It should not be supported
for 32-bit CPUs.

[1] https://s3-eu-west-1.amazonaws.com/downloads-mips/documents/MD00087-2B-MIPS64BIS-AFP-6.06.pdf

llvm-svn: 363886