[SVE][CodeGen] Lower legal integer -> floating point conversions

This patch adds new ISD nodes, SCVTZ_MERGE_PASSTHRU &
UCVTZ_MERGE_PASSTHRU, which are used to lower both legal
scalable vector [S|U]INT_TO_FP operations and the following intrinsics:
 - llvm.aarch64.sve.scvtf
 - llvm.aarch64.sve.ucvtf

Reviewed By: sdesmalen, efriedma

Differential Revision: https://reviews.llvm.org/D87913