TIntermBinary::promote() was incorrectly marking the type of result as const in some cases. The result can only be const if both operands are const. Also cleaned up the function to remove redundant/repeated checks.
BUG=24
TEST=OpenGL ES 2.0 Conformance tests
Review URL: http://codereview.appspot.com/1938047

git-svn-id: https://angleproject.googlecode.com/svn/trunk@385 736b8ea6-26fd-11df-bfd4-992fa37f6226