conditionalget: avoid const lookup in case/when

case/when dispatches already optimize away allocation of constant
string literals in all C Ruby 1.9.x/2.x releases
(ref: opt_case_dispatch in Ruby insns.def)

Other Ruby implementations presumably have similar optimizations
to encourage prettier code.

The following code snippet does not cause GC.count to increase
during the two loops, regardless of what `nr' is.
Tested on Ruby 1.9.3, 2.1.3, and trunk r47786:

    GET = "GET"
    HEAD = "HEAD"
    REQUEST_METHOD = "REQUEST_METHOD" # unnecessary in 2.2.0+
    env = { REQUEST_METHOD => "GET" }

    nr = 10000000
    nr.times do |i|
      case env[REQUEST_METHOD]
      when GET, HEAD
        :foo
      else
        :bar
      end
    end
    a = GC.count

    nr.times do |i|
      case env[REQUEST_METHOD]
      when "GET", "HEAD"
        :foo
      else
        :bar
      end
    end
    b = GC.count
    p [ a, b ]