glsl: Clamp vector indices when lowering to swizzles

glsl: Clamp vector indices when lowering to swizzles

This prevents other code from seeing a swizzle of the 16th component
of a vector, for example.

NOTE: This is a candidate for the 7.11 branch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=42517
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Tested-by: Christian Holler <choller@mozilla.com>