transform: optimize range over []byte(string)

Fixes #2700

transform/testdata/stringtobytes.ll

@@ -5,6 +5,8 @@ target triple = "x86_64--linux"
 
 declare { ptr, i64, i64 } @runtime.stringToBytes(ptr, i64)
 
+declare void @printByte(i8)
+
 declare void @printSlice(ptr nocapture readonly, i64, i64)
 
 declare void @writeToSlice(ptr nocapture, i64, i64)
@@ -17,6 +19,12 @@ entry:
   %2 = extractvalue { ptr, i64, i64 } %0, 1
   %3 = extractvalue { ptr, i64, i64 } %0, 2
   call fastcc void @printSlice(ptr %1, i64 %2, i64 %3)
+
+  ; print(slice[0])
+  %indexaddr.ptr1 = extractvalue { ptr, i64, i64 } %0, 0
+  %4 = getelementptr inbounds i8, ptr %indexaddr.ptr1, i64 0
+  %5 = load i8, ptr %4, align 1
+  call fastcc void @printByte(i8 %5)
   ret void
 }
 

transform/testdata/stringtobytes.out.ll

@@ -5,6 +5,8 @@ target triple = "x86_64--linux"
 
 declare { ptr, i64, i64 } @runtime.stringToBytes(ptr, i64)
 
+declare void @printByte(i8)
+
 declare void @printSlice(ptr nocapture readonly, i64, i64)
 
 declare void @writeToSlice(ptr nocapture, i64, i64)
@@ -12,6 +14,9 @@ declare void @writeToSlice(ptr nocapture, i64, i64)
 define void @testReadOnly() {
 entry:
   call fastcc void @printSlice(ptr @str, i64 6, i64 6)
+  %0 = getelementptr inbounds i8, ptr @str, i64 0
+  %1 = load i8, ptr %0, align 1
+  call fastcc void @printByte(i8 %1)
   ret void
 }
 

transform/util.go

@@ -38,15 +38,18 @@ func hasFlag(call, param llvm.Value, kind string) bool {
 func isReadOnly(value llvm.Value) bool {
 	uses := getUses(value)
 	for _, use := range uses {
-		if !use.IsAGetElementPtrInst().IsNil() {
+		switch {
+		case !use.IsAGetElementPtrInst().IsNil():
 			if !isReadOnly(use) {
 				return false
 			}
-		} else if !use.IsACallInst().IsNil() {
+		case !use.IsACallInst().IsNil():
 			if !hasFlag(use, value, "readonly") {
 				return false
 			}
-		} else {
+		case !use.IsALoadInst().IsNil():
+			// Loads are read-only.
+		default:
 			// Unknown instruction, might not be readonly.
 			return false
 		}