S/390: Merge movdi_larl into movdi_64

Consider the following RTL:

(insn (set (mem/f/c:DI (reg/f:DI 60))
           (const:DI (plus:DI (symbol_ref:DI ("*.LANCHOR0"))
                              (const_int 8)))))

generated by cse2 pass.  It is matched to movdi_64, resulting in
the following inefficient code:

	larl	%r5,.L6			# Load literal pool@
	lg	%r1,.L7-.L6(%r5)	# Load .LANCHOR0+8
	stgrl	%r1,.LANCHOR0
	br	%r14

Matching it to movdi_larl improves the code, eliminating one
instruction and the literal pool entry:

	larl	%r1,.LANCHOR0+8
	stgrl	%r1,.LANCHOR0
	br	%r14

Taking it one step further, there is no reason to keep movdi_64 and
movdi_larl separate, since this could potentially improve code in other
ways by giving lra one more alternative to choose from.

gcc/ChangeLog:

2018-10-25  Ilya Leoshkevich  <iii@linux.ibm.com>

	* config/s390/constraints.md (ZL): New constraint.
	* config/s390/s390.c (legitimate_pic_operand_p): Accept LARL
	operands.
	* config/s390/s390.md (movdi_larl): Remove.
	(movdi_64): Add the LARL alternative.

gcc/testsuite/ChangeLog:

2018-10-25  Ilya Leoshkevich  <iii@linux.ibm.com>

	* gcc.target/s390/global-array-almost-huge-element.c: New test.
	* gcc.target/s390/global-array-almost-negative-huge-element.c: New test.
	* gcc.target/s390/global-array-element-pic.c: New test.
	* gcc.target/s390/global-array-even-element.c: New test.
	* gcc.target/s390/global-array-huge-element.c: New test.
	* gcc.target/s390/global-array-negative-huge-element.c: New test.
	* gcc.target/s390/global-array-odd-element.c: New test.

From-SVN: r265490