[X86] Prevent promotion of i16 add/sub/and/or/xor to i32 if we can fold an...

[X86] Prevent promotion of i16 add/sub/and/or/xor to i32 if we can fold an atomic load and atomic store.

This makes them consistent with i8/i32/i64. Which still seems to be more aggressive on folding than icc, gcc, or MSVC.

llvm-svn: 338795