Simplify (truncate:QI (subreg:SI (reg:QI x))) to (reg:QI x)

Whilst working on a backend patch, I noticed that the middle-end's
RTL optimizers weren't simplifying a truncation of a paradoxical
subreg extension, though it does transform closely related (more
complex) expressions.  The main (first) part of this patch
implements this simplification, reusing much of the logic already
in place.

I briefly considered suggesting that it's difficult to provide a new
testcase for this change, but then realized the reviewer's response
would be that this type of transformation should be self-tested
in simplify-rtx, so this patch adds a bunch of tests that integer
extensions and truncations are simplified as expected.  No good
deed goes unpunished and I was equally surprised to see that we
don't currently simplify/check/defend (zero_extend:SI (reg:SI)),
i.e. useless no-op extensions to the same mode.  So I've added
some logic to simplify (or more accurately prevent us generating
dubious RTL for) those.

2021-08-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* simplify-rtx.c (simplify_truncation): Generalize simplification
	of (truncate:A (subreg:B X)).
	(simplify_unary_operation_1) [FLOAT_TRUNCATE, FLOAT_EXTEND,
	SIGN_EXTEND, ZERO_EXTEND]: Handle cases where the operand
	already has the desired machine mode.
	(test_scalar_int_ops): Add tests that useless extensions and
	truncations are optimized away.
	(test_scalar_int_ext_ops): New self-test function to confirm
	that truncations of extensions are correctly simplified.
	(test_scalar_int_ext_ops2): New self-test function to check
	truncations of truncations, extensions of extensions, and
	truncations of extensions.
	(test_scalar_ops): Call the above two functions with a
	representative sampling of integer machine modes.

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