nir: check NIR_SKIP to skip passes by name

nir: check NIR_SKIP to skip passes by name

Passes' function names, separated by comma, listed in NIR_SKIP
environment variable will be skipped in debug mode.  The mechanism is
hooked into the _PASS macro, like NIR_PRINT.

The extra macro NIR_SKIP is available as a developer convenience, to
skip at pointer other than the passes entry points.

v2: Fix typo in NIR_SKIP macro. (Bas)

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>