mode-switching: Add a target-configurable confluence operator

The mode-switching pass assumed that all of an entity's modes
were mutually exclusive.  However, the upcoming SME changes
have an entity with some overlapping modes, so that there is
sometimes a "superunion" mode that contains two given modes.
We can use this relationship to pass something more helpful than
"don't know" to the emit hook.

This patch adds a new hook that targets can use to specify
a mode confluence operator.

With mutually exclusive modes, it's possible to compute a block's
incoming and outgoing modes by looking at its availability sets.
With the confluence operator, we instead need to solve a full
dataflow problem.

However, when emitting a mode transition, the upcoming SME use of
mode-switching benefits from having as much information as possible
about the starting mode.  Calculating this information is definitely
worth the compile time.

The dataflow problem is written to work before and after the LCM
problem has been solved.  A later patch makes use of this.

While there (since git blame would ping me for the reindented code),
I used a lambda to avoid the cut-&-pasted loops.

gcc/
	* target.def (mode_switching.confluence): New hook.
	* doc/tm.texi (TARGET_MODE_CONFLUENCE): New @hook.
	* doc/tm.texi.in: Regenerate.
	* mode-switching.cc (confluence_info): New variable.
	(mode_confluence, forward_confluence_n, forward_transfer): New
	functions.
	(optimize_mode_switching): Use them to calculate mode_in when
	TARGET_MODE_CONFLUENCE is defined.

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