[X86] Part 1 to fix x86-64 fp128 calling convention.

[X86] Part 1 to fix x86-64 fp128 calling convention.

Almost all these changes are conditioned and only apply to the new
x86-64 f128 type configuration, which will be enabled in a follow up
patch. They are required together to make new f128 work. If there is
any error, we should fix or revert them as a whole.
These changes should have no impact to current configurations.

* Relax type legalization checks to accept new f128 type configuration,
  whose TypeAction is TypeSoftenFloat, not TypeLegal, but also has
  TLI.isTypeLegal true.
* Relax GetSoftenedFloat to return in some cases f128 type SDValue,
  which is TLI.isTypeLegal but not "softened" to i128 node.
* Allow customized FABS, FNEG, FCOPYSIGN on new f128 type configuration,
  to generate optimized bitwise operators for libm functions.
* Enhance related Lower* functions to handle f128 type.
* Enhance DAGTypeLegalizer::run, SoftenFloatResult, and related functions
  to keep new f128 type in register, and convert f128 operators to library calls.
* Fix Combiner, Emitter, Legalizer routines that did not handle f128 type.
* Add ExpandConstant to handle i128 constants, ExpandNode
  to handle ISD::Constant node.
* Add one more parameter to getCommonSubClass and firstCommonClass,
  to guarantee that returned common sub class will contain the specified
  simple value type.
  This extra parameter is used by EmitCopyFromReg in InstrEmitter.cpp.
* Fix infinite loop in getTypeLegalizationCost when f128 is the value type.
* Fix printOperand to handle null operand.
* Enhance ISD::BITCAST node to handle f128 constant.
* Expand new f128 type for BR_CC, SELECT_CC, SELECT, SETCC nodes.
* Enhance X86AsmPrinter to emit f128 values in comments.

Differential Revision: http://reviews.llvm.org/D15134

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254653 91177308-0d34-0410-b5e6-96231b3b80d8