PrelRules: Don't violate let/app invariant
Previously shiftRule would rewrite as invalid shift like
let x = I# (uncheckedIShiftL# n 80)
in ...to
let x = I# (error "invalid shift")
in ...However, this breaks the let/app invariant as error is not
okay-for-speculation. There isn't an easy way to avoid this so let's not
try. Instead we just take advantage of the undefined nature of invalid
shifts and return zero.
Fixes #16742 (closed).