[X86] Part 2 to fix x86-64 fp128 calling convention.

Part 1 was submitted in http://reviews.llvm.org/D15134.
Changes in this part:
* X86RegisterInfo.td, X86RecognizableInstr.cpp: Add FR128 register class.
* X86CallingConv.td: Pass f128 values in XMM registers or on stack.
* X86InstrCompiler.td, X86InstrInfo.td, X86InstrSSE.td:
  Add instruction selection patterns for f128.
* X86ISelLowering.cpp:
  When target has MMX registers, configure MVT::f128 in FR128RegClass,
  with TypeSoftenFloat action, and custom actions for some opcodes.
  Add missed cases of MVT::f128 in places that handle f32, f64, or vector types.
  Add TODO comment to support f128 type in inline assembly code.
* SelectionDAGBuilder.cpp:
  Fix infinite loop when f128 type can have
  VT == TLI.getTypeToTransformTo(Ctx, VT).
* Add unit tests for x86-64 fp128 type.

Differential Revision: http://reviews.llvm.org/D11438

llvm-svn: 255558