Kill off the 'coerce' ABI passing form. Now 'direct' and 'extend' always

have a "coerce to" type which often matches the default lowering of Clang
type to LLVM IR type, but the coerce case can be handled by making them
not be the same.

This simplifies things and fixes issues where X86-64 abi lowering would 
return coerce after making preferred types exactly match up.  This caused
us to compile:

typedef float v4f32 __attribute__((__vector_size__(16)));
v4f32 foo(v4f32 X) {
  return X+X;
}

into this code at -O0:

define <4 x float> @foo(<4 x float> %X.coerce) nounwind {
entry:
  %retval = alloca <4 x float>, align 16          ; <<4 x float>*> [#uses=2]
  %coerce = alloca <4 x float>, align 16          ; <<4 x float>*> [#uses=2]
  %X.addr = alloca <4 x float>, align 16          ; <<4 x float>*> [#uses=3]
  store <4 x float> %X.coerce, <4 x float>* %coerce
  %X = load <4 x float>* %coerce                  ; <<4 x float>> [#uses=1]
  store <4 x float> %X, <4 x float>* %X.addr
  %tmp = load <4 x float>* %X.addr                ; <<4 x float>> [#uses=1]
  %tmp1 = load <4 x float>* %X.addr               ; <<4 x float>> [#uses=1]
  %add = fadd <4 x float> %tmp, %tmp1             ; <<4 x float>> [#uses=1]
  store <4 x float> %add, <4 x float>* %retval
  %0 = load <4 x float>* %retval                  ; <<4 x float>> [#uses=1]
  ret <4 x float> %0
}

Now we get:

define <4 x float> @foo(<4 x float> %X) nounwind {
entry:
  %X.addr = alloca <4 x float>, align 16          ; <<4 x float>*> [#uses=3]
  store <4 x float> %X, <4 x float>* %X.addr
  %tmp = load <4 x float>* %X.addr                ; <<4 x float>> [#uses=1]
  %tmp1 = load <4 x float>* %X.addr               ; <<4 x float>> [#uses=1]
  %add = fadd <4 x float> %tmp, %tmp1             ; <<4 x float>> [#uses=1]
  ret <4 x float> %add
}

This implements rdar://8248065



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@109733 91177308-0d34-0410-b5e6-96231b3b80d8

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