[mips][microMIPS] Fix an issue with selecting sqrt instruction in LLVM backend

Summary:
This fixes 7 tests during fast LLVM test-suite run:
* MultiSource/Benchmarks/McCat/18-imp/imp
* MultiSource/Applications/oggenc/oggenc
* MultiSource/Benchmarks/MallocBench/gs/gs
* MultiSource/Benchmarks/MiBench/automotive-susan/automotive-susan
* MultiSource/Benchmarks/VersaBench/beamformer/beamformer
* MultiSource/Benchmarks/MiBench/consumer-lame/consumer-lame
* MultiSource/Benchmarks/Bullet/bullet

Error message was in the form of:
fatal error: error in backend: Cannot select: 0x95c3288: f32 = fsqrt 0x95c0190 [ORD=9] [ID=18]
  0x95c0190: f32 = fadd 0x95bef30, 0x95c4d00 [ORD=8] [ID=17]
    0x95bef30: f32 = fmul 0x95c4988, 0x95c4988 [ORD=5] [ID=16]
...

There was problem with selecting sqrt instruction in LLVM backend.

To fix the issue changes are made in TableGen definition for sqrt instruction in MipsInstrFPU.td and new test file sqrt.ll is added to LLVM regression tests.

Patch by Zlatko Buljan

Reviewers: zoran.jovanovic, hvarga, dsanders

Subscribers: llvm-commits, petarj

Differential Revision: http://reviews.llvm.org/D13235

llvm-svn: 249416