[SVE] Fixed custom lowering of ISD::INSERT_SUBVECTOR.

LowerINSERT_SUBVECTOR emits AArch64ISD::UUNPK## when lowering
scalable vector floating point INSERT_SUBVECTOR. However, these
nodes only make sense for integer types and thus isel patterns do
not exist for floating point, which leads to isel failures.

This patch ensures floating point operands are cast to integer
before the core lowering takes place.

Fixes: #55037

Differential Revision: https://reviews.llvm.org/D126487