JIT: Don't assume the reserved bits in GQRs are zero

While the unused bits in the GQR SPR are probably not supposed to be set, some
games set them anyways (e.g. Dirt 2), which broke the JIT code.
For better clarity, I suggest changing the comment to  "Some games (e.g. Dirt 2) incorrectly set the unused bits which breaks the lookup table code. Hence, we need to mask out the unused bits by ANDing with 0x3F07."