[InstCombine] transform more extract/insert pairs into shuffles (PR2109)

This is an extension of the shuffle combining from r203229:
http://reviews.llvm.org/rL203229

The idea is to widen a short input vector with undef elements so the
existing shuffle transform for extract/insert can kick in.

The motivation is to finally solve PR2109:
https://llvm.org/bugs/show_bug.cgi?id=2109

For that example, the IR becomes:

%1 = bitcast <2 x i32>* %P to <2 x float>*
%ld1 = load <2 x float>, <2 x float>* %1, align 8
%2 = shufflevector <2 x float> %ld1, <2 x float> undef, <4 x i32> <i32 0, i32 1, i32 undef, i32 undef>
%i2 = shufflevector <4 x float> %A, <4 x float> %2, <4 x i32> <i32 0, i32 1, i32 4, i32 5>
ret <4 x float> %i2

And x86 SSE output improves from:

movq	(%rdi), %xmm1           ## xmm1 = mem[0],zero
movdqa	%xmm1, %xmm2
shufps	$229, %xmm2, %xmm2      ## xmm2 = xmm2[1,1,2,3]
shufps	$48, %xmm0, %xmm1       ## xmm1 = xmm1[0,0],xmm0[3,0]
shufps	$132, %xmm1, %xmm0      ## xmm0 = xmm0[0,1],xmm1[0,2]
shufps	$32, %xmm0, %xmm2       ## xmm2 = xmm2[0,0],xmm0[2,0]
shufps	$36, %xmm2, %xmm0       ## xmm0 = xmm0[0,1],xmm2[2,0]
retq

To the almost optimal:

movhpd	(%rdi), %xmm0

Note: There's a tension in the existing transform related to generating
arbitrary shufflevector masks. We avoid that in other places in InstCombine
because we're scared that codegen can't handle strange masks, but it looks
like we're ok with producing those here. I purposely chose weird insert/extract
indexes for the regression tests to see the effect in these cases. 
For PowerPC+Altivec, AArch64, and X86+SSE/AVX, I think the codegen is equal or
better for these examples.

Differential Revision: http://reviews.llvm.org/D15096

llvm-svn: 256394