[AArch64][SVE] Fix missed immediate selection due to mishandling of signedness

The complex selection pattern for add/sub shifted immediates is
incorrect in it's handling of incoming constant values, in that it
does not properly anticipate the values to be signed extended to
32-bits.

Co-authored-by: Graham Hunter <graham.hunter@arm.com>

Differential Revision: https://reviews.llvm.org/D101833