make the argument passing stuff in the FCA case smarter still, by

make the argument passing stuff in the FCA case smarter still, by
avoiding making the FCA at all when the types exactly line up.  For
example, before we made:

%struct.DeclGroup = type { i64, i64 }

define i64 @_Z3foo9DeclGroup(i64, i64) nounwind {
entry:
  %D = alloca %struct.DeclGroup, align 8          ; <%struct.DeclGroup*> [#uses=3]
  %2 = insertvalue %struct.DeclGroup undef, i64 %0, 0 ; <%struct.DeclGroup> [#uses=1]
  %3 = insertvalue %struct.DeclGroup %2, i64 %1, 1 ; <%struct.DeclGroup> [#uses=1]
  store %struct.DeclGroup %3, %struct.DeclGroup* %D
  %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 ; <i64*> [#uses=1]
  %tmp3 = load i64* %tmp2                         ; <i64> [#uses=1]
  %add = add nsw i64 %tmp1, %tmp3                 ; <i64> [#uses=1]
  ret i64 %add
}

... which has the pointless insertvalue, which fastisel hates, now we
make:

%struct.DeclGroup = type { i64, i64 }

define i64 @_Z3foo9DeclGroup(i64, i64) nounwind {
entry:
  %D = alloca %struct.DeclGroup, align 8          ; <%struct.DeclGroup*> [#uses=4]
  %2 = getelementptr %struct.DeclGroup* %D, i32 0, i32 0 ; <i64*> [#uses=1]
  store i64 %0, i64* %2
  %3 = getelementptr %struct.DeclGroup* %D, i32 0, i32 1 ; <i64*> [#uses=1]
  store i64 %1, i64* %3
  %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 ; <i64*> [#uses=1]
  %tmp3 = load i64* %tmp2                         ; <i64> [#uses=1]
  %add = add nsw i64 %tmp1, %tmp3                 ; <i64> [#uses=1]
  ret i64 %add
}

This only kicks in when x86-64 abi lowering decides it likes us.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107104 91177308-0d34-0410-b5e6-96231b3b80d8