[DAGCombine] Skip re-visiting EntryToken to avoid compile time explosion

During the main DAGCombine loop, whenever a node gets replaced, the new
node and all its users are pushed onto the worklist.  Omit this if the
new node is the EntryToken (e.g. if a store managed to get optimized
out), because re-visiting the EntryToken and its users will not uncover
any additional opportunities, but there may be a large number of such
users, potentially causing compile time explosion.

This compile time explosion showed up in particular when building the
SingleSource/UnitTests/matrix-types-spec.cpp test-suite case on any
platform without SIMD vector support.

Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D86963