transform: wasm-abi: create temporary allocas in the entry block

This avoids problems with goroutines in WebAssembly, and is generally a
good thing. It fixes some cases of the following problem:

    LLVM ERROR: Coroutines cannot handle non static allocas yet

Author: aykevl

transform/testdata/wasm-abi.out.ll

@@ -14,11 +14,11 @@ define internal i64 @testCall(i8* %ptr, i32 %len, i64 %foo) {
 
 define internal i64 @testCallNonEntry(i8* %ptr, i32 %len) {
 entry:
+  %i64asptr = alloca i64
+  %i64asptr1 = alloca i64
   br label %bb1
 
 bb1:                                              ; preds = %entry
-  %i64asptr = alloca i64
-  %i64asptr1 = alloca i64
   store i64 3, i64* %i64asptr1
   call void @externalCall(i64* %i64asptr, i8* %ptr, i32 %len, i64* %i64asptr1)
   %retval = load i64, i64* %i64asptr

transform/wasm-abi.go

@@ -22,6 +22,11 @@ func ExternalInt64AsPtr(mod llvm.Module) error {
 	int64Type := ctx.Int64Type()
 	int64PtrType := llvm.PointerType(int64Type, 0)
 
+	// This builder is only used for creating new allocas in the entry block of
+	// a function, avoiding many SetInsertPoint* calls.
+	entryBlockBuilder := ctx.NewBuilder()
+	defer entryBlockBuilder.Dispose()
+
 	for fn := mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
 		if fn.Linkage() != llvm.ExternalLinkage {
 			// Only change externally visible functions (exports and imports).
@@ -77,19 +82,20 @@ func ExternalInt64AsPtr(mod llvm.Module) error {
 			// be left in place.
 			for use := fn.FirstUse(); !use.IsNil(); use = use.NextUse() {
 				call := use.User()
+				entryBlockBuilder.SetInsertPointBefore(call.InstructionParent().Parent().EntryBasicBlock().FirstInstruction())
 				builder.SetInsertPointBefore(call)
 				callParams := []llvm.Value{}
 				var retvalAlloca llvm.Value
 				if fnType.ReturnType() == int64Type {
-					retvalAlloca = builder.CreateAlloca(int64Type, "i64asptr")
+					retvalAlloca = entryBlockBuilder.CreateAlloca(int64Type, "i64asptr")
 					callParams = append(callParams, retvalAlloca)
 				}
 				for i := 0; i < call.OperandsCount()-1; i++ {
 					operand := call.Operand(i)
 					if operand.Type() == int64Type {
 						// Pass a stack-allocated pointer instead of the value
 						// itself.
-						alloca := builder.CreateAlloca(int64Type, "i64asptr")
+						alloca := entryBlockBuilder.CreateAlloca(int64Type, "i64asptr")
 						builder.CreateStore(operand, alloca)
 						callParams = append(callParams, alloca)
 					} else {