[AArch64][SVE] Bail out of performPostLD1Combine for scalable types

Summary:
performPostLD1Combine will introduce either a LD1LANEpost
or LD1DUPpost node, which will cause selection failure if the
return type is a scalable vector.

Reviewers: sdesmalen, c-rhodes, efriedma

Reviewed By: efriedma

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, danielkiss, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82670