[VE] (fp)trunc+store & load+(fp)ext isel

Summary: load+sext/zext/fpext and (fp)trunc+store isel legalization and tests

Reviewers: arsenm, craig.topper, rengolin, k-ishizaka

Reviewed By: arsenm

Subscribers: merge_guards_bot, wdng, hiraditya, llvm-commits

Tags: #ve, #llvm

Differential Revision: https://reviews.llvm.org/D73774