[x86] use shuffles for scalar insertion into high elements of a constant vector

As discussed in D54073, we have a potential regression from more aggressive vector narrowing here, so let's try to avoid that by changing build-vector lowering slightly.

Insert-vector-element lowering always does this since there's no "pinsr" for ymm/zmm:

// If the vector is wider than 128 bits, extract the 128-bit subvector, insert
// into that, and then insert the subvector back into the result.

...but we can sometimes do better for insert-into-constant-vector by using shuffle lowering.

Differential Revision: https://reviews.llvm.org/D54271

llvm-svn: 346433

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