ignore structs that wrap vectors in IR, the abstraction shouldn't add penalty.

Before we'd compile the example into something like:

  %coerce.dive2 = getelementptr %struct.v4f32wrapper* %retval, i32 0, i32 0 ; <<4 x float>*> [#uses=1]
  %1 = bitcast <4 x float>* %coerce.dive2 to <2 x double>* ; <<2 x double>*> [#uses=1]
  %2 = load <2 x double>* %1, align 1             ; <<2 x double>> [#uses=1]
  ret <2 x double> %2

Now we produce:

  %coerce.dive2 = getelementptr %struct.v4f32wrapper* %retval, i32 0, i32 0 ; <<4 x float>*> [#uses=1]
  %0 = load <4 x float>* %coerce.dive2, align 1   ; <<4 x float>> [#uses=1]
  ret <4 x float> %0

llvm-svn: 109732