[SPARC][MC] Fix encoding of backwards BPr branches

Make sure that the upper bits of the offset is placed in bits 20-21 of the
instruction word.

This fixes the encoding of backwards (negative offset) BPr branches.

(Previously, the upper two bits of the offset would overwrite parts of the rs1
field, causing it to branch on the wrong register, with the wrong offset)

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D144012