[AArch64][DAGCombine] Fix a bug in performBuildVectorCombine where it could produce...

[AArch64][DAGCombine] Fix a bug in performBuildVectorCombine where it could produce an invalid EXTRACT_SUBVECTOR

EXTRACT_SUBVECTOR requires that Idx be a constant multiple of ResultType's
known minimum vector length.

Something like this will produce an invalid extract_subvector:

t1: v4i16 = .....
t2: i32 = extract_vector_elt t1, Constant:i64<1>
t3: i32 = extract_vector_elt t1, Constant:i64<2>
t4: v2i32 = BUILD_VECTOR t2, t3
// produces
t5: v2i32 = extract_subvector t...., Constant:i64<1>

Differential Revision: https://reviews.llvm.org/D132517

Change-Id: I7a5acf054edee3e89c0f85a28d8869256403ce08