[SDAG] narrow truncated sign_extend_inreg

trunc (sign_ext_inreg X, iM) to iN --> sign_ext_inreg (trunc X to iN), iM

There are improvements on existing tests from this, and there are a pair
of large regressions in D127115 for Thumb2 caused by not folding this
pattern.

Differential Revision: https://reviews.llvm.org/D129890