[mips] Parse the 'bopt' and 'nobopt' directives in IAS.

The GAS assembler supports the ".set bopt" directive but according
to the sources it doesn't do anything. It's supposed to optimize
branches by filling the delay slot of a branch with it's target.

This patch teaches the MIPS asm parser to accept both and warn in
the case of 'bopt' that the bopt directive is unsupported.

This resolves PR/31841.

Thanks to Sean Bruno for reporting the issue!

llvm-svn: 293798