[AArch64][SVE] Fix calculation restore point for SVE callee saves.

This fixes an issue where the restore point of callee-saves in the
function epilogues was incorrectly calculated when the basic block
consisted of only a RET instruction. This caused dealloc instructions
to be inserted in between the block of callee-save restore instructions,
rather than before it.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D86099