[RISCV] Support constant operand for la and lla pseudoinstruction.

This patch improves compatibility with GNU assembler by adding support for
constant immediate in la and lla pseudo instruction, and expanding it in the
same way as we currently expands li pseudo instruction.

Links to discussion related to the above issue in the community -
https://github.com/riscv-non-isa/riscv-arch-test/issues/105
https://github.com/riscv-non-isa/riscv-arch-test/issues/108
https://github.com/riscv-non-isa/riscv-arch-test/issues/106

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D150133