[Committed] Restore build on !TARGET_TRULY_NOOP_TRUNCATION targets

[Committed] Restore build on !TARGET_TRULY_NOOP_TRUNCATION targets

[Roger Sayle]

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My sincere apologies to everyone, but especially Andrew Pinski
who warned me in advance that TRULY_NOOP_TRUNCATION results in
different code paths/optimizations on some targets. This restores
the build on nvptx-none (and presumably others) where mysteriously
(truncate:QI (reg:QI)) fails to be simplified to (reg:QI), which
is expected (everywhere) in my recently added self-tests.
[I'd even tested the functional change of the previous patch on
nvptx-none, but it's the new self-tests that I added for submission
that have been shown to be less portable across targets than hoped].

This patch has been committed as obvious, after testing that it
restores the build on nvptx-none and with a "make bootstrap" on
x86_64-pc-linux-gnu to guarantee no surprises [make -k check is
underway].

Sorry again.

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

gcc/ChangeLog
	* simplify-rtx.c (simplify_unary_operation_1): [TRUNCATE]:
	Handle case where the operand is already the desired mode.

Roger
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diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index f3df614..8eea9fb 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -1268,6 +1268,9 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
 	    return temp;
 	}
 
+      /* Check for useless truncation.  */
+      if (GET_MODE (op) == mode)
+	return op;
       break;
 
     case FLOAT_TRUNCATE: