[AArch64] Match float round and convert to int instructions.

Summary:
Add patterns for doing floating point round with various rounding modes
followed by conversion to int as a single FCVT* instruction.

Reviewers: t.p.northover, jmolloy

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D11424

llvm-svn: 243422