[mips] Make whitespace in disassembler tests more consistent. NFC.

The tests for the ISA's should now be approximately diffable. That is, the
output of 'diff valid-mips1.txt valid-mips2.txt' should be emit the lines
for instructions that were added/removed to/from MIPS-I by MIPS-II. This
doesn't work perfectly at the moment due to ordering differences but it
should be close.

llvm-svn: 226408