[WebAssembly] Add patterns for relaxed madd (#147487)
[WebAssembly] Fold fadd contract (fmul contract) to relaxed madd w/
-mattr=+simd128,+relaxed-simd
Fixes #121311
- Precommit test for #121311
- Fold fadd contract (fmul contract) to relaxed madd w/
-mattr=+simd128,+relaxed-simd
- Move PatFrag of fadd_contract in ARM.td and WebAssembly.td to
TargetSelectionDAG.td for reuse of pattern
