ubsan: alpha-vms: shift exponent 536874240 is too large

ubsan: alpha-vms: shift exponent 536874240 is too large

C_OPR_ASH is supposed to be an arithmetic shift.  By the look of it,
this operator implemented logical shifts since the original binutils
support was added.  This patch corrects that and avoids some nonsense
ubsan complaints.  I chose to implement infinite precision shifts
rather than masking shift counts to the word size as the spec I had is
silent on what is supposed to happen with overlarge shift counts.

	* vms-alpha.c (_bfd_vms_slurp_etir <ETIR__C_OPR_ASH>): Implement
	shifts without undefined behaviour.