[SelectionDAG][AVR] Add support for lrint and lround intrinsics

Integer legalization already supported splitting the output integer of
llround and llrint, but did not support this for lround and lrint yet.
This is not a problem for 32-bit architectures, but for 8/16-bit
architectures like AVR it results in a crash like this:

    ExpandIntegerResult #0: t7: i32 = lround t6

    LLVM ERROR: Do not know how to expand the result of this operator!

This patch simply add lrint/lround to the list of ISD opcodes to expand.

Fixes https://github.com/llvm/llvm-project/issues/59573.

Differential Revision: https://reviews.llvm.org/D140822