read/write_pieced_value: Remove unnecessary variable copies

read/write_pieced_value: Remove unnecessary variable copies

In read_pieced_value's main loop, the variables `dest_offset_bits' and
`source_offset_bits' are basically just copies of `offset' and
`bits_to_skip', respectively.  In write_pieced_value the copies are
reversed.  This is not very helpful when trying to keep the logic between
these functions in sync.  Since the copies are unnecessary, this patch
just removes them.

gdb/ChangeLog:

	* dwarf2loc.c (read_pieced_value): Remove unnecessary variables
	dest_offset_bits and source_offset_bits.
	(write_pieced_value): Likewise.