compiler,runtime: use LLVM intrinsics for memcpy/memmove

This replaces the custom runtime.memcpy and runtime.memmove functions
with calls to LLVM builtins that should hopefully allow LLVM to better
optimize such calls. They will be lowered to regular libc memcpy/memmove
when they can't be optimized away.

When testing this change with some smoke tests, I found that many smoke
tests resulted in slightly larger binary sizes with this commit applied.
I looked into it and it appears that machine.sendUSBPacket was not
inlined before while it is with this commit applied. Additionally, when
I compared all driver smoke tests with -opt=1 I saw that many were
reduced slightly in binary size and none increased in size.

compiler/compiler.go

@@ -330,7 +330,6 @@ func Compile(pkgName string, machine llvm.TargetMachine, config *compileopts.Con
 		return c.ctx.CreateEnumAttribute(attrKind, 0)
 	}
 	nocapture := getAttr("nocapture")
-	writeonly := getAttr("writeonly")
 	readonly := getAttr("readonly")
 
 	// Tell the optimizer that runtime.alloc is an allocator, meaning that it
@@ -354,16 +353,6 @@ func Compile(pkgName string, machine llvm.TargetMachine, config *compileopts.Con
 		trackPointer.AddAttributeAtIndex(1, readonly)
 	}
 
-	// Memory copy operations do not capture pointers, even though some weird
-	// pointer arithmetic is happening in the Go implementation.
-	for _, fnName := range []string{"runtime.memcpy", "runtime.memmove"} {
-		fn := c.mod.NamedFunction(fnName)
-		fn.AddAttributeAtIndex(1, nocapture)
-		fn.AddAttributeAtIndex(1, writeonly)
-		fn.AddAttributeAtIndex(2, nocapture)
-		fn.AddAttributeAtIndex(2, readonly)
-	}
-
 	// see: https://reviews.llvm.org/D18355
 	if c.Debug() {
 		c.mod.AddNamedMetadataOperand("llvm.module.flags",
@@ -1352,6 +1341,8 @@ func (b *builder) createFunctionCall(instr *ssa.CallCommon) (llvm.Value, error)
 		// applied) function call. If it is anonymous, it may be a closure.
 		name := fn.RelString(nil)
 		switch {
+		case name == "runtime.memcpy" || name == "runtime.memmove" || name == "reflect.memcpy":
+			return b.createMemoryCopyCall(fn, instr.Args)
 		case name == "device/arm.ReadRegister" || name == "device/riscv.ReadRegister":
 			return b.createReadRegister(name, instr.Args)
 		case name == "device/arm.Asm" || name == "device/avr.Asm" || name == "device/riscv.Asm":