[MSP430] Declare comparison LibCalls as returning i16 instead of i32

For TI's distribution of msp430-gcc
```
msp430-elf-gcc -S -o- -Os -x c - <<< "int f(float a, float b) { return a != b; }"
```
does not mention `R13` at all. `__libgcc_cmp_return__` machine mode is 2 byte on MSP430, as well.

Differential Revision: https://reviews.llvm.org/D82635