Add fptrunc to mips fast-sel

Summary: Implement conversion of 64 to 32 bit floating point numbers (fptrunc) in mips fast-isel

Test Plan:
fptrunc.ll
checked also with 4 internal mips build bot flavors mip32r1/miprs32r2 and at -O0 and -O2

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: rfuhler

Differential Revision: http://reviews.llvm.org/D5553