[CostModel][AArch64] Make extractelement, with fmul user, free whenev… (#111479)

…er possible

In case of Neon, if there exists extractelement from lane != 0 such that
  1. extractelement does not necessitate a move from vector_reg -> GPR
  2. extractelement result feeds into fmul
3. Other operand of fmul is a scalar or extractelement from lane 0 or
lane equivalent to 0
then the extractelement can be merged with fmul in the backend and it
incurs no cost.

  e.g. 
  ```
define double @foo(<2 x double> %a) { 
    %1 = extractelement <2 x double> %a, i32 0 
    %2 = extractelement <2 x double> %a, i32 1
    %res = fmul double %1, %2    
    ret double %res
  }
```
  `%2` and `%res` can be merged in the backend to generate:
  `fmul    d0, d0, v0.d[1]`

The change was tested with SPEC FP(C/C++) on Neoverse-v2. 
**Compile time impact**: None
**Performance impact**: Observing 1.3-1.7% uplift on lbm benchmark with -flto depending upon the config.

0 files changed, 0 insertions, 0 deletions