[mips] Switch the MIPS-III and MIPS-IV assembler tests to use -mcpu=mips4.

Summary:
It is now the smallest superset for these ISA's.

FeatureMips4 now contains FeatureFPIdx since [ls][dw]xc1 were added in MIPS-IV.
Made the FPIdx feature bit lowercase so that it can be used in the -mattr option.

Depends on D3274

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://reviews.llvm.org/D3275

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205964 91177308-0d34-0410-b5e6-96231b3b80d8