codegen/llvm: fix memset with vectors smaller than one byte
The special case to take advantage of llvm's intrinsic
was generating invalid llvm ir:
```
Invalid bitcast
%60 = bitcast <2 x i2> %59 to i8, !dbg !3122
thread 145453 panic: LLVM module verification failed
```
5ab1854602Xavier Bouchoux committed on 7/26/2023, 1:14:10 AM· committed by Andrew Kelleyparent71dfce3