[x64] Zero/sign-extend loads to 64-bit registers.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index 45380465d4b8e42dd479e36ece009ef9efcbf826..9a0d18e9c2fba4c71648b999eb560165927ddbf9 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -1586,6 +1586,13 @@ void Assembler::movsxbq(Register dst, const Operand& src) {
   emit_operand(dst, src);
 }
 
+void Assembler::movsxbq(Register dst, Register src) {
+  EnsureSpace ensure_space(this);
+  emit_rex_64(dst, src);
+  emit(0x0F);
+  emit(0xBE);
+  emit_modrm(dst, src);
+}
 
 void Assembler::movsxwl(Register dst, Register src) {
   EnsureSpace ensure_space(this);
@@ -1613,6 +1620,13 @@ void Assembler::movsxwq(Register dst, const Operand& src) {
   emit_operand(dst, src);
 }
 
+void Assembler::movsxwq(Register dst, Register src) {
+  EnsureSpace ensure_space(this);
+  emit_rex_64(dst, src);
+  emit(0x0F);
+  emit(0xBF);
+  emit_modrm(dst, src);
+}
 
 void Assembler::movsxlq(Register dst, Register src) {
   EnsureSpace ensure_space(this);