Shifting a signed 1 into the sign bit is undefined, and UBSan reports it
as "left shift of 1 by 31 places cannot be represented in type 'int'".
The switch-flag helpers in z_actor.c take the bit position at runtime, so
every flag from 0x00 to 0x3F goes through the same shift. Flag 0x3F lands
on bit 31: the Potion Shop Granny sets and clears it while brewing the Odd
Potion, so this is reached in ordinary play, not just in theory.
The same shift appears as a constant in gBitFlags, in the two player state
flags that occupy bit 31, and in ENHORSE_FLAG_31.
Suffixing the 1 with u moves the shift to unsigned, where the value is
defined. Every flag field involved is already u32, so the results are
unchanged.