[llvm] [VPlan] Introduces explicit broadcast for live-in constants. (PR #133213)

[Elvis Wang via llvm-commits]

https://github.com/ElvisWang123 commented:

Sorry didn't notice the previous comment. 

For follow-up optimizations, if the `materializeBroadcasts()` could move before vplan-based register pressure calculation. I will tried to make the register usage of the constant splat can be calculated. 

Although many targets support folding constant value into one operand in vector instructions i.e. `.vx` instructions in RISCV. But not all instructions support this kind of folding. 
Explicit showing constants broadcast on the vplan can help estimate the register pressure more accurately.


https://github.com/llvm/llvm-project/pull/133213