PR tree-optimization/101403: Incorrect folding of ((T)bswap(x))>>C

PR tree-optimization/101403: Incorrect folding of ((T)bswap(x))>>C

My sincere apologies for the breakage.  My recent patch to fold
bswapN(x)>>C where the constant C was large enough that the result
only contains bits from the low byte, and can therefore avoid
the byte swap contains a minor logic error.  The pattern contains
a convert? allowing an extension to occur between the bswap and
the shift.  The logic is correct if there's no extension, or the
extension has the same sign as the shift, but I'd mistakenly
convinced myself that these couldn't have different signedness.

(T)bswap16(x)>>12 is (T)((unsigned char)x>>4) or (T)((signed char)x>>4).
The bug is that for zero-extensions to signed type T, we need to use
the unsigned char variant [the signedness of the byte shift is not
(always) the same as the signedness of T and the original shift].

Then because I'm now paranoid, I've also added a clause to handle
the hypothetical (but in practice impossible) sign-extension to an
unsigned type T, which can implemented as (T)(x<<8)>>12.

2021-07-12  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR tree-optimization/101403
	* match.pd ((T)bswap(X)>>C): Correctly handle cases where
	signedness of the shift is not the same as the signedness of
	the type extension.

gcc/testsuite/ChangeLog
	PR tree-optimization/101403
	* gcc.dg/pr101403.c: New test case.