Change X86_64ABIInfo to have ASTContext and TargetData ivars to

Change X86_64ABIInfo to have ASTContext and TargetData ivars to
avoid passing ASTContext down through all the methods it has.

When classifying an argument, or argument piece, as INTEGER, check
to see if we have a pointer at exactly the same offset in the
preferred type.  If so, use that pointer type instead of i64.  This
allows us to compile A function taking a stringref into something
like this:

define i8* @foo(i64 %D.coerce0, i8* %D.coerce1) nounwind ssp {
entry:
  %D = alloca %struct.DeclGroup, align 8          ; <%struct.DeclGroup*> [#uses=4]
  %0 = getelementptr %struct.DeclGroup* %D, i32 0, i32 0 ; <i64*> [#uses=1]
  store i64 %D.coerce0, i64* %0
  %1 = getelementptr %struct.DeclGroup* %D, i32 0, i32 1 ; <i8**> [#uses=1]
  store i8* %D.coerce1, i8** %1
  %tmp = getelementptr inbounds %struct.DeclGroup* %D, i32 0, i32 0 ; <i64*> [#uses=1]
  %tmp1 = load i64* %tmp                          ; <i64> [#uses=1]
  %tmp2 = getelementptr inbounds %struct.DeclGroup* %D, i32 0, i32 1 ; <i8**> [#uses=1]
  %tmp3 = load i8** %tmp2                         ; <i8*> [#uses=1]
  %add.ptr = getelementptr inbounds i8* %tmp3, i64 %tmp1 ; <i8*> [#uses=1]
  ret i8* %add.ptr
}

instead of this:

define i8* @foo(i64 %D.coerce0, i64 %D.coerce1) nounwind ssp {
entry:
  %D = alloca %struct.DeclGroup, align 8          ; <%struct.DeclGroup*> [#uses=3]
  %0 = insertvalue %0 undef, i64 %D.coerce0, 0    ; <%0> [#uses=1]
  %1 = insertvalue %0 %0, i64 %D.coerce1, 1       ; <%0> [#uses=1]
  %2 = bitcast %struct.DeclGroup* %D to %0*       ; <%0*> [#uses=1]
  store %0 %1, %0* %2, align 1
  %tmp = getelementptr inbounds %struct.DeclGroup* %D, i32 0, i32 0 ; <i64*> [#uses=1]
  %tmp1 = load i64* %tmp                          ; <i64> [#uses=1]
  %tmp2 = getelementptr inbounds %struct.DeclGroup* %D, i32 0, i32 1 ; <i8**> [#uses=1]
  %tmp3 = load i8** %tmp2                         ; <i8*> [#uses=1]
  %add.ptr = getelementptr inbounds i8* %tmp3, i64 %tmp1 ; <i8*> [#uses=1]
  ret i8* %add.ptr
}

This implements rdar://7375902 - [codegen quality] clang x86-64 ABI lowering code punishing StringRef

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107123 91177308-0d34-0410-b5e6-96231b3b80d8