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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Using the smaller EEW reduces both the total number of instructions, and the number of vector uops, on implementations that split on the size of the vector.  And on an implementation which splits on the number of elements, we'd need a bunch of other tweaks to generate good code, anyway.  (For example, we'd want to rewrite i8 vector popcount to use i64 operations.)

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