[GCC/ARM, 2/2] Add support for ASRL(imm), LSLL(imm) and LSRL(imm) instructions for Armv8.1-M Mainline

This patch is adding the following instructions:

ASRL (imm)
LSLL (imm)
LSRL (imm)

*** gcc/ChangeLog ***

2020-01-17  Mihail-Calin Ionescu  <mihail.ionescu@arm.com>
	    Sudakshina Das  <sudi.das@arm.com>

	* config/arm/arm.md (ashldi3): Generate thumb2_lsll for both reg
	and valid immediate.
	(ashrdi3): Generate thumb2_asrl for both reg and valid immediate.
	(lshrdi3): Generate thumb2_lsrl for valid immediates.
	* config/arm/constraints.md (Pg): New.
	* config/arm/predicates.md (long_shift_imm): New.
	(arm_reg_or_long_shift_imm): Likewise.
	* config/arm/thumb2.md (thumb2_asrl): New immediate alternative.
	(thumb2_lsll): Likewise.
	(thumb2_lsrl): New.

*** gcc/testsuite/ChangeLog ***

2020-01-17  Mihail-Calin Ionescu  <mihail.ionescu@arm.com>
	    Sudakshina Das  <sudi.das@arm.com>

	* gcc.target/arm/armv8_1m-shift-imm_1.c: New test.