glsl: add is_lhs bool on ast_expression

glsl: add is_lhs bool on ast_expression

Useful to know if a expression is the recipient of an assignment
or not, that would be used to (for example) raise warnings of
"use of uninitialized variable" without getting a false positive
when assigning first a variable.

By default the value is false, and it is assigned to true on
the following cases:
 * The lhs assignments subexpression
 * At ast_array_index, on the array itself.
 * While handling the method on an array, to avoid the warning
   calling array.length
 * When computed the cached test expression at test_to_hir, to
   avoid a duplicate warning on the test expression of a switch.

set_is_lhs setter is added, because in some cases (like ast_field_selection)
the value need to be propagated on the expression tree. To avoid doing the
propatagion if not needed, it skips if no primary_expression.identifier is
available.

v2: use a new bool on ast_expression, instead of a new parameter
    on ast_expression::hir (Timothy Arceri)

v3: fix style and some typos on comments, initialize is_lhs default value
    on constructor, to avoid a c++11 feature (Ian Romanick)

v4: some tweaks on comments (Timothy Arceri)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94129

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>