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

This patch adds new ISD nodes, FCVTZS_MERGE_PASSTHRU &
FCVTZU_MERGE_PASSTHRU, which are used to lower scalable vector
FP_TO_SINT/FP_TO_UINT operations and the following intrinsics:
 - llvm.aarch64.sve.fcvtzu
 - llvm.aarch64.sve.fcvtzs

Reviewed By: efriedma, paulwalker-arm

Differential Revision: https://reviews.llvm.org/D87232