MSP430: Enable relaxation of jump instructions to hard-coded pcrel offsets

MSP430: Enable relaxation of jump instructions to hard-coded pcrel offsets

This patch fixes execution failures which occur when the BR in a
sequence such as:
  J<cond> 1f
  BR
  1:
is relaxed to a JMP, and the pc-relative offset for the destination of
the J<cond> instruction is hard-coded to be 2 words ahead of the
instruction.
The hard-coded offset will cause execution to jump 1 word ahead of where
it should actually go.

Instead we now detect the hard-coded offset is one we inserted earlier,
and invert the condition, allowing us to remove the BR entirely.

bfd/ChangeLog:

2020-02-10  Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	* elf32-msp430.c (msp430_elf_relax_section): Before relaxing a branch,
	check if previous instruction matches a conditional jump inserted
	earlier. Invert conditional jump and delete branch in this case.

(cherry picked from commit ac4280dad035bb98fba999c174314435aefdf1a0)