Fixing the handling of DW_OP_bregx.

According to the DWARF4 spec:
The DW_OP_bregx operation has two operands: a register which is
specified by an unsigned LEB128 number, followed by a signed LEB128
offset.

Issue: PLCR-545
Merge remote-tracking branch 'origin/bugfix/PLCR-545-dw_op_bregx-is-handled-incorrectly'