compiler: removes unnecessary code paths (#2266)

This removes the unnecessary code paths in a various places,
and the below is the result:

```
goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
                      │  old.txt   │             new.txt              │
                      │   sec/op   │   sec/op    vs base              │
Compilation/wazero-10   1.634 ± 0%   1.626 ± 0%  -0.51% (p=0.002 n=6)
Compilation/zig-10      3.588 ± 0%   3.538 ± 2%       ~ (p=0.065 n=6)
Compilation/zz-10       15.25 ± 0%   14.87 ± 1%  -2.46% (p=0.002 n=6)
geomean                 4.472        4.406       -1.46%

                      │   old.txt    │              new.txt               │
                      │     B/op     │     B/op      vs base              │
Compilation/wazero-10   271.2Mi ± 0%   271.2Mi ± 0%       ~ (p=1.000 n=6)
Compilation/zig-10      596.3Mi ± 0%   596.3Mi ± 0%       ~ (p=0.699 n=6)
Compilation/zz-10       528.9Mi ± 0%   528.9Mi ± 0%       ~ (p=0.818 n=6)
geomean                 440.6Mi        440.6Mi       +0.00%

                      │   old.txt   │              new.txt              │
                      │  allocs/op  │  allocs/op   vs base              │
Compilation/wazero-10   448.5k ± 0%   448.5k ± 0%       ~ (p=0.937 n=6)
Compilation/zig-10      274.8k ± 0%   274.7k ± 0%       ~ (p=1.000 n=6)
Compilation/zz-10       618.3k ± 0%   618.4k ± 0%       ~ (p=0.818 n=6)
geomean                 423.9k        423.9k       -0.00%
```


Signed-off-by: Takeshi Yoneda <[email protected]>

internal/engine/wazevo/backend/regalloc/regalloc.go

@@ -229,9 +229,6 @@ func (s *state[I, B]) getVRegState(v VRegID) *vrState[I, B] {
 }
 
 func (s *state[I, B]) useRealReg(r RealReg, vr *vrState[I, B]) {
-	if s.regsInUse.has(r) {
-		panic("BUG: useRealReg: the given real register is already used")
-	}
 	s.regsInUse.add(r, vr)
 	vr.r = r
 	s.allocatedRegSet = s.allocatedRegSet.add(r)
@@ -250,16 +247,16 @@ func (s *state[I, B]) releaseRealReg(r RealReg) {
 func (vs *vrState[I, B]) recordReload(f Function[I, B], blk B) {
 	vs.spilled = true
 	var nilBlk B
-	if vs.lca == nilBlk {
+	if lca := vs.lca; lca == nilBlk {
 		if wazevoapi.RegAllocLoggingEnabled {
 			fmt.Printf("\t\tv%d is reloaded in blk%d,\n", vs.v.ID(), blk.ID())
 		}
 		vs.lca = blk
-	} else {
+	} else if lca != blk {
 		if wazevoapi.RegAllocLoggingEnabled {
 			fmt.Printf("\t\tv%d is reloaded in blk%d, lca=%d\n", vs.v.ID(), blk.ID(), vs.lca.ID())
 		}
-		vs.lca = f.LowestCommonAncestor(vs.lca, blk)
+		vs.lca = f.LowestCommonAncestor(lca, blk)
 		if wazevoapi.RegAllocLoggingEnabled {
 			fmt.Printf("updated lca=%d\n", vs.lca.ID())
 		}

internal/engine/wazevo/ssa/builder.go

@@ -294,10 +294,6 @@ func (b *builder) Init(s *Signature) {
 	b.nextValueID = 0
 	b.reversePostOrderedBasicBlocks = b.reversePostOrderedBasicBlocks[:0]
 	b.doneBlockLayout = false
-	for i := range b.valueRefCounts {
-		b.valueRefCounts[i] = 0
-	}
-
 	b.currentSourceOffset = sourceOffsetUnknown
 }