i965: Delete completely bogus comment

i965: Delete completely bogus comment

This hasn't been true in 6+ years, if it was even true then.  Before
we rewrote the compiler and introduced GLSL IR in 2010-2011, i965 used
to have two compiler backends for WM programs, based on Mesa IR.  One
handled flow control and was SIMD8-only, while the other was SIMD16
only and didn't handle flow control.  Or something like that.

Even then, this certainly didn't handle vertex shaders, so "all ...
code generation" is a bit strong.