R600: Use legacy (0 * anything = 0) MUL instructions for pow intrinsics

Fixes wrong lighting in some corner cases with r600g and radeonsi, e.g.
manifested by failure of two piglit/glean tests and intermittent black
patches in many apps.

Tested on SI and RS880.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=62012 [radeonsi]
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58150 [r600g]

NOTE: This is a candidate for the Mesa stable branch.

Reviewed-by: Christian König <christian.koenig@amd.com>