TOutputGLSLBase::visitLoop clarify the function slightly
TOutputGLSLBase::visitLoop clarify the function slightly

Since only for loops can be unrolled, handle each type of loop only
once. This improvement was going to be part of a do-while workaround for
Mac that will instead be done at the AST level.

BUG=angleproject:891

Change-Id: Ic8ecf41aa6d9abd7a01c2230e908c85152f52b62
Reviewed-on: https://chromium-review.googlesource.com/302332
Tryjob-Request: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Tested-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

mikolalysenko/angle,ppy/angle,mikolalysenko/angle,ppy/angle,mlfarrell/angle,MSOpenTech/angle,domokit/waterfall,mybios/angle,ppy/angle,MSOpenTech/angle,ecoal95/angle,mikolalysenko/angle,ecoal95/angle,ppy/angle,mybios/angle,mikolalysenko/angle,ecoal95/angle,MSOpenTech/angle,mybios/angle,domokit/waterfall,ecoal95/angle,mlfarrell/angle,mlfarrell/angle,ecoal95/angle,MSOpenTech/angle,mlfarrell/angle,mybios/angle