[Bug target/46091] missed optimization: x86 bt/btc/bts instructions

["ubizjak at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46091

--- Comment #7 from Uroš Bizjak <ubizjak at gmail dot com> ---
From i386.md:

;; %%% bts, btr, btc, bt.
;; In general these instructions are *slow* when applied to memory,
;; since they enforce atomic operation.  When applied to registers,
;; it depends on the cpu implementation.  They're never faster than
;; the corresponding and/ior/xor operations, so with 32-bit there's
;; no point.  But in 64-bit, we can't hold the relevant immediates
;; within the instruction itself, so operating on bits in the high
;; 32-bits of a register becomes easier.