[trans-mem] PR47606: Add a tm-safe marker for GIMPLE_ASM's

[Aldy Hernandez <aldyh at redhat dot com>]

This is yet another problem with inline assembly in transactions.

The problem here is that when we diagnose the presence of inline 
assembly in diagnose_tm(), inlining has not yet run.  After many 
transformations, we may end up inlining GIMPLE_ASM's into a transaction, 
but we lost the context in which it appeared.

In this testcase we have a GIMPLE_ASM in a transaction_pure function, so 
it should be allowed in a transaction when inlined, instead of ICEing 
when seeing the statement later.

I bit the bullet and added a subcode bit to the GIMPLE_ASM tuple to keep 
track that the ASM appeared in a TM pure function.  Richard, I believe 
you had a similar idea, by looking at the comment I removed below.

This patch fixes the testcase in the original PR, though I found yet 
another GIMPLE_ASM bug when playing around with variants of the 
testcase.  I can attack that next, as it's the same ICE but a different 
problem.

OK for branch?

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