[AArch64][SME] Fix LDR and STR asm parser

The LDR and STR instructions must have the same value for imm4(second operand)
and offset(fourth operand).
The disassembly guarantees that happens, but the Asm parser was not checking that.
This patch fixes that by checking if the second operand and fourth operand are
immediate and have the same value.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D147617