x86: Replace "is not" with "!=" in fpop.isa.

Some variables were being compared against some constants with "is not",
which is not correct since it will compare for identity rather than
equivalence. There was a long standing build warning from this, but it
wasn't clear where the error was coming from since it was in python
interpreted from a string in the ISA description.

This change replaces "is not" in those two places with "!=".

Change-Id: I0c4d038af6e047ffd79f8171713e8e998e840e3b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33283
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>

src/arch/x86/isa/microops/fpop.isa

@@ -123,7 +123,7 @@ let {{
 
             # If there's something optional to do with flags, generate
             # a version without it and fix up this version to use it.
-            if flag_code is not "" or cond_check is not "true":
+            if flag_code != "" or cond_check != "true":
                 self.buildCppClasses(name, Name, suffix,
                         code, "", "true", else_code, op_class)
                 suffix = "Flags" + suffix