[SCCP] Simplify CFG in SCCP as well

Currently, we only remove dead blocks and non-feasible edges in
IPSCCP, but not in SCCP. I'm not aware of any strong reason for
that difference, so this patch updates SCCP to perform the CFG
cleanup as well.

Compile-time impact seems to be pretty minimal, in the 0.05%
geomean range on CTMark.

For the test case from https://reviews.llvm.org/D126962#3611579
the result after -sccp now looks like this:

    define void @test(i1 %c) {
    entry:
      br i1 %c, label %unreachable, label %next
    next:
      unreachable
    unreachable:
      call void @bar()
      unreachable
    }

-jump-threading does nothing on this, but -simplifycfg will produce
the optimal result.

Differential Revision: https://reviews.llvm.org/D128796