glsl: fix array assignments of a swizzled vector

This happens in situations where we might do

  vec.wzyx[i] = ...

The swizzle would get effectively ignored because of the interaction
between how ir_assignment->set_lhs works and overwriting the write_mask.
There are two cases, one where i is a constant, and another where i is
variable. We have to be extra-careful in both cases.

Fixes the following WebGL test:

  https://www.khronos.org/registry/webgl/sdk/tests/conformance2/glsl3/vector-dynamic-indexing-swizzled-lvalue.html



And the new piglit tests:

  swizzled-writemask-indexing-nonconst.shader_test
  swizzled-writemask-indexing.shader_test

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 1bb1c03d)