This was actually ack'd late in the gcc-14 cycle, but I chose not to 
integrate it given how late we were in the cycle.

The basic idea here is to track liveness of subobjects within a word and 
if we find an extension where the bits set aren't actually used, then we 
convert the extension into a subreg.  The subreg typically simplifies away.

I've seen this help a few routines in coremark, fix one bug in the 
testsuite (pr111384) and fix a couple internally reported bugs in Ventana.

The original idea and code were from Joern; Jivan and I hacked it into 
usable shape.  I've had this in my tester for ~8 months, so it's been 
through more build/test cycles than I care to contemplate and nearly 
every architecture we support.

But just in case, I'm going to wait for it to spin through the 
pre-commit CI tester.  I'll find my old ChangeLog before committing.

Jeff