[mips] Fix sync instruction definition

The 'sync' instruction for MIPS was defined in MIPS-II as taking no operands.
MIPS32 extended the define of 'sync' as taking an optional unsigned 5 bit
immediate.

This patch correct the definition of sync so that it is accepted with an
operand of 0 or no operand for MIPS-II to MIPS-V, and a 5 bit unsigned
immediate for MIPS32 and later revisions.

Additionally a clear error is given when the MIPS32 version of sync is
used when targeting pre MIPS32.

This partially resolves PR/30714.

Thanks to Daniel Sanders for reporting this issue!

Reveiwers: vkalintiris

Differential Revision: https://reviews.llvm.org/D25672

llvm-svn: 284483