[Bug target/70119] AArch64 should take advantage of implicit truncation of variable shift amount without defining SHIFT_COUNT_TRUNCATED

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70119

--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Richard Biener from comment #1)
> Confirmed.  SHIFT_COUNT_TRUNCATED should simply go away.

The same limitation is with x86. Shifts trucate the shift count, but bit-test
(BT) instructions take modulo of the bit offset operand. We have somehow hacked
around this by introducing <shift><mode>3_mask patterns to i386.md in the hope
that combine catches the masking, but the solution is far from optimal.