[patch] PR middle-end/54385

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

Hello,

With my patch to visit insns in topological order in
init_alias_analysis, an issue in postreload-cse got exposed: Blocks
may become unreachable between calls of reload_cse_regs_1 but no CFG
cleanups are scheduled between them, resulting in an ICE in
pre_and_rev_post_order_compute at an assert that all blocks have been
visited in the depth first search on the CFG.

This patch fixes the problem by deleting unreachable basic blocks if
edges were purged during a reload-cse transformation on an insn. I
didn't add a test case because reliably triggering problems this late
in the pass pipeline is so hard.

Bootstrapped&tested on powerpc64-unknown-linux-gnu. OK for trunk?

Ciao!
Steven

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