rs6000: Wrap diff of immediates in const (PR83629)

rs6000: Wrap diff of immediates in const (PR83629)

In various of our 32-bit load_toc patterns we take the difference of
two immediates (labels) as a term to something bigger; but this isn't
canonical RTL, it needs to be wrapped in CONST.

	PR target/83629
	* config/rs6000/rs6000.md (load_toc_v4_PIC_2, load_toc_v4_PIC_3b,
	load_toc_v4_PIC_3c): Wrap const term in CONST RTL.

testsuite/
	PR target/83629
	* gcc.target/powerpc/pr83629.c: New testcase.

From-SVN: r256432