x64: use constant pool for u64 constants rather than movabs. (#4088)

* Allow emitting u64 constants into constant pool.

* Use constant pool for constants on x64 that do not fit in a simm32 and are needed as a RegMem or RegMemImm.

* Fix rip-relative addressing bug in pinsrd emission.

cranelift/codegen/src/machinst/isle.rs

@@ -539,6 +539,12 @@ macro_rules! isle_prelude_methods {
             let offset: i32 = offset.into();
             offset as u32
         }
+
+        #[inline]
+        fn emit_u64_le_const(&mut self, value: u64) -> VCodeConstant {
+            let data = VCodeConstantData::U64(value.to_le_bytes());
+            self.lower_ctx.use_constant(data)
+        }
     };
 }