Generate more efficient code for zero or sign extensions where
the source is a subvector generated via SHUFFLE_VECTOR.
Specifically, recognize patterns corresponding to (series of)
VECTOR UNPACK instructions, or the VECTOR SIGN EXTEND TO
DOUBLEWORD instruction.
As a special case, also handle zero or sign extensions of a
vector element to i128.
Fixes: https://github.com/llvm/llvm-project/issues/129576
Fixes: https://github.com/llvm/llvm-project/issues/129899
