compiler: truncate string slice indices if necessary

This didn't trigger on most platforms but does trigger on AVR where
almost all slice operations on strings are with integers that are bigger
than uintptr.

compiler/compiler.go

@@ -1973,6 +1973,8 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 
 			c.emitSliceBoundsCheck(frame, llvmLen, low, high, lowType, highType)
 
+			// Truncate ints bigger than uintptr. This is after the bounds
+			// check so it's safe.
 			if c.targetData.TypeAllocSize(high.Type()) > c.targetData.TypeAllocSize(c.uintptrType) {
 				high = c.builder.CreateTrunc(high, c.uintptrType, "")
 			}
@@ -2005,6 +2007,8 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 
 			c.emitSliceBoundsCheck(frame, oldCap, low, high, lowType, highType)
 
+			// Truncate ints bigger than uintptr. This is after the bounds
+			// check so it's safe.
 			if c.targetData.TypeAllocSize(low.Type()) > c.targetData.TypeAllocSize(c.uintptrType) {
 				low = c.builder.CreateTrunc(low, c.uintptrType, "")
 			}
@@ -2038,6 +2042,15 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 
 			c.emitSliceBoundsCheck(frame, oldLen, low, high, lowType, highType)
 
+			// Truncate ints bigger than uintptr. This is after the bounds
+			// check so it's safe.
+			if c.targetData.TypeAllocSize(low.Type()) > c.targetData.TypeAllocSize(c.uintptrType) {
+				low = c.builder.CreateTrunc(low, c.uintptrType, "")
+			}
+			if c.targetData.TypeAllocSize(high.Type()) > c.targetData.TypeAllocSize(c.uintptrType) {
+				high = c.builder.CreateTrunc(high, c.uintptrType, "")
+			}
+
 			newPtr := c.builder.CreateGEP(oldPtr, []llvm.Value{low}, "")
 			newLen := c.builder.CreateSub(high, low, "")
 			str, err := c.getZeroValue(c.mod.GetTypeByName("runtime._string"))