[DSE] Preserve basic block ordering using OrderedBasicBlock.

By extending OrderedBB to allow removing and replacing cached
instructions, we can preserve OrderedBBs in DSE easily. This eliminates
one source of quadratic compile time in DSE.

Fixes PR38829.

Reviewers: rnk, efriedma, hfinkel

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D59789

llvm-svn: 357208