Fix signed immediate operand bug for the 2 branch instruction kinds (Bx ...

Sorry there was a little misunderstanding on my part in my previous commit (I will work on the test script for the future).
There's 2 kinds of PPC branch instruction that have a signed immediate operand: Bx and Bcx.
The signed operand has a different encoding: In Bx it's 24 bits long, in Bcx it's 14 bits long.