Remove wrongly repeated words in comments

Clean up misrepetitions ('if if', 'the the' etc) found throughout the
comments. This has been done manually, after grepping
case-insensitively for duplicate if, is, the, then, do, for, an,
plus a few other typos corrected in fly-by

v2:
    * proper commit message and non-joke title;
    * replace two 'as is' followed by 'is' to 'as-is'.
v3:
    * 'a integer' => 'an integer' and similar (originally spotted by
      Jason Ekstrand, I fixed a few other similar ones while at it)

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>

src/compiler/nir/nir_intrinsics.h

@@ -48,9 +48,9 @@ INTRINSIC(copy_var, 0, ARR(0), false, 0, 2, 0, xx, xx, xx, 0)
 
 /*
  * Interpolation of input.  The interp_var_at* intrinsics are similar to the
- * load_var intrinsic acting an a shader input except that they interpolate
+ * load_var intrinsic acting on a shader input except that they interpolate
  * the input differently.  The at_sample and at_offset intrinsics take an
- * aditional source that is a integer sample id or a vec2 position offset
+ * additional source that is an integer sample id or a vec2 position offset
  * respectively.
  */