Reduce time and memory usage of GVN.

Filter out dead sregs in GVN. Reclaim memory after each LVN
in the GVN modification phase.

Bug: 16398693

(cherry picked from commit b19955d3c8fbd9588f7e17299e559d02938154b6)

Change-Id: I33c7912258a768b4c99d787056979fbc3b023b3b
10 files changed