[AArch64][SVE] Remove erroneous assert in resolveFrameOffsetReference

The code already supports addressing a fixed-size stack object from
the frame-pointer, by first subtracting sizeof(SVE area) from FP.

Reviewers: efriedma, cameron.mcinally, david-arm, rengolin

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D83125