[AArch64][GlobalISel] Form G_DUPLANE32 for <2 x s32> shufflevectors in lowering.

For <2 x s32>, we can use G_DUPLANE32, but with a <4 x s32> source. To make it
work, we can just widen the original source with a concat_vectors.

Doing this allows <2 x float> indexed fmul instruction selection patterns to
fire, which gives a nice 0.3% code size saving on Bullet with -Os.

Differential Revision: https://reviews.llvm.org/D98059