[llvm] [RISCV] Use QC_E_LI to materialise addresses (PR #155819)

[Sam Elliott via llvm-commits]

lenary wrote:

@topperc it would be good to understand your feelings about this (even without a full review). I've been wary of changing the materialisation sequences so far, but this gets us a measurable code size improvement - customer images are seeing a 6-12% improvement (one image with 170k of code is seeing a 9% improvement, so this is not just an artefact of small images).

These numbers are using our linker which has relaxations for `qc.e.li` (to `qc.li`, `c.lui`, or a gp-relative `addi`) - but not relaxations that turn `lui+addi` into `qc.e.li` or something smaller (which end up needing lots of constraints, including what effectively is a one-use check on the `lui`). 

https://github.com/llvm/llvm-project/pull/155819