Rename EvqConst to EvqConstExpr

In GLSL ES a variable declared const is a compile-time constant, and
thus a constant expression. Parameters declared const are read-only but
not compile-time constant.

Bug 19511934

Change-Id: I6d02062ca62c4185a3384b31a4d3ce8dfcc86b35
Reviewed-on: https://swiftshader-review.googlesource.com/2441
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>