[mips][microMIPS] Disable FastISel for microMIPS

Author: milena.vujosevic.janicic
Reviewers: dsanders

FastIsel is not supported for microMIPS, thus it needs to be disabled. 
Test micromips-zero-mat-uses.ll is deleted since the tested sequence of instructions is not generated for microMIPS without FastISel.
Differential Revision: http://reviews.llvm.org/D15892

llvm-svn: 259039