Tracks the first subpass to touch an attachment.

Test: dEQP-VK.renderpass.suballocation.unused*
Change-Id: Idc0998da022fd4c0b1785f6020c795916ba475f7
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31308Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>