[mips][microMIPS] Fix an invalid read for lwm32 and reserved reglist values.

Summary:
Some values of 'reglist' are reserved and cause the disassembler to read past
the end of the Regs array. Treat lwm32's containing reserved values as invalid
instructions.

Reviewers: zoran.jovanovic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12959

llvm-svn: 247990