The code that patches the line raster emulation directive assumed both
vertex and fragment shaders would be present.  This is invalid if only a
vertex shader is present.

This change adds the directive to any stage that's available.  This
includes the possibility of a missing vertex shader also, which can be
useful for separable shaders and program pipeline objects.

Bug: angleproject:3394
Change-Id: I84555db81b59c318300003f7108a01f330e4259f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1982782
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>