[llvm] [RISCV] Construct constants via instructions if materialization is costly (PR #86926)

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

Only vector part is included and we don't optimize if LMUL==M8 as it increases register preasure. Now there is no noticeable regression.

https://github.com/llvm/llvm-project/pull/86926