glsl: Lower variable indexing of system value arrays unconditionally.

lower_variable_index_to_cond_assign() did not handle system values.
gl_SampleMaskIn[] is a system value, and also an array.  Accessing it
with a variable index would trigger an unreachable() assert.

Rather than adding a new EmitNoIndirectSystemValues flag, we simply
lower unconditionally.  There is exactly one case where this occurs,
and for all current drivers, lowering produces optimal code.  Even
for future drivers with 32x MSAA, it produces reasonable code.

Fixes Piglit's new samplemaskin-indirect test.  Also fixes many ES31-CTS
tests when OES_sample_variables is enabled.

Cc: [email protected]
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ilia Mirkin <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>

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