[APFloat] Switch from (PPCDoubleDoubleImpl, IEEEdouble) layout to (IEEEdouble, IEEEdouble)

Summary:
This patch changes the layout of DoubleAPFloat, and adjust all
operations to do either:
1) (IEEEdouble, IEEEdouble) -> (uint64_t, uint64_t) -> PPCDoubleDoubleImpl,
   then run the old algorithm.
2) Do the right thing directly.

1) includes multiply, divide, remainder, mod, fusedMultiplyAdd, roundToIntegral,
   convertFromString, next, convertToInteger, convertFromAPInt,
   convertFromSignExtendedInteger, convertFromZeroExtendedInteger,
   convertToHexString, toString, getExactInverse.
2) includes makeZero, makeLargest, makeSmallest, makeSmallestNormalized,
   compare, bitwiseIsEqual, bitcastToAPInt, isDenormal, isSmallest,
   isLargest, isInteger, ilogb, scalbn, frexp, hash_value, Profile.

I could split this into two patches, e.g. use
1) for all operatoins first, then incrementally change some of them to
2). I didn't do that, because 1) involves code that converts data between
PPCDoubleDoubleImpl and (IEEEdouble, IEEEdouble) back and forth, and may
pessimize the compiler. Instead, I find easy functions and use
approach 2) for them directly.

Next step is to implement move multiply and divide from 1) to 2). I don't
have plans for other functions in 1).

Differential Revision: https://reviews.llvm.org/D27872

llvm-svn: 292839

Author: timshen91

clang/test/CodeGen/ppc64-complex-parms.c

@@ -93,7 +93,7 @@ void bar_long_double(void) {
 // CHECK: %[[VAR22:[A-Za-z0-9.]+]] = getelementptr inbounds { ppc_fp128, ppc_fp128 }, { ppc_fp128, ppc_fp128 }* %[[VAR21]], i32 0, i32 0
 // CHECK: %[[VAR23:[A-Za-z0-9.]+]] = getelementptr inbounds { ppc_fp128, ppc_fp128 }, { ppc_fp128, ppc_fp128 }* %[[VAR21]], i32 0, i32 1
 // CHECK: store ppc_fp128 0xM40000000000000000000000000000000, ppc_fp128* %[[VAR22]]
-// CHECK: store ppc_fp128 0xMC0040000000000000000000000000000, ppc_fp128* %[[VAR23]]
+// CHECK: store ppc_fp128 0xMC0040000000000008000000000000000, ppc_fp128* %[[VAR23]]
 // CHECK: %[[VAR24:[A-Za-z0-9.]+]] = getelementptr inbounds { ppc_fp128, ppc_fp128 }, { ppc_fp128, ppc_fp128 }* %[[VAR21]], i32 0, i32 0
 // CHECK: %[[VAR25:[A-Za-z0-9.]+]] = load ppc_fp128, ppc_fp128* %[[VAR24]], align 16
 // CHECK: %[[VAR26:[A-Za-z0-9.]+]] = getelementptr inbounds { ppc_fp128, ppc_fp128 }, { ppc_fp128, ppc_fp128 }* %[[VAR21]], i32 0, i32 1