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

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>