x86: `%xmm` registers are not completely highlighted when the number differs

[1superchip]

In this scratch https://decomp.me/scratch/yYjWc, we see diffs where the number of the %xmm register differs but the entire register is not colored.

The left side has %xmm2 where only 2 is colored while the right side has %xmm0 where only 0 is colored.

A regex probably needs to be updated to include the numbers in xmm

[1superchip]

@simonlindholm pointed out that the PPC immediate regex (re_imm) has (\b|-) at the beginning of it whereas the current x86 immediate regex just has -? at the beginning.

Replacing -? with (\b|-) in the x86 immediate regex correctly highlights the entire xmm register

I will get this change pr'ed after checking other objects to ensure the regex change does not cause any issues.