This required a new instruction group representing the 32-bit subset of
MIPS-III that was available in MIPS32
A small number of instructions are correctly rejected but with the wrong error
message. These have been placed in a separate test for now.
There's some obvious InstAlias's that ought to be marked MIPS-III but arent.
This is because they are not currently tested. I intend to catch these with
a final pass through the tablegen records to find tablegen records without
ISA annotations.
Depends on D3674