glsl: Rename "expr" to "lhs_expr" in vector_extract munging code.
glsl: Rename "expr" to "lhs_expr" in vector_extract munging code.

When processing assignments, we have both an LHS and RHS.  At a glance,
"lhs_expr" clearly refers to the LHS, while a generic name like "expr"
is ambiguous.

Cc: 59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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