Fix alloca builder creation (#107464)

This was hitting assert with debug version of llvm

src/mono/mono/mini/mini-llvm.c

@@ -2962,7 +2962,10 @@ build_alloca_llvm_type_name (EmitContext *ctx, LLVMTypeRef t, int align, const c
 	 * Have to place all alloca's at the end of the entry bb, since otherwise they would
 	 * get executed every time control reaches them.
 	 */
-	LLVMPositionBuilder (ctx->alloca_builder, get_bb (ctx, ctx->cfg->bb_entry), ctx->last_alloca);
+	if (ctx->last_alloca)
+		LLVMPositionBuilder (ctx->alloca_builder, get_bb (ctx, ctx->cfg->bb_entry), ctx->last_alloca);
+	else
+		LLVMPositionBuilderAtEnd (ctx->alloca_builder, get_bb (ctx, ctx->cfg->bb_entry));
 
 	ctx->last_alloca = mono_llvm_build_alloca (ctx->alloca_builder, t, NULL, align, name);
 	return ctx->last_alloca;