Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Re-introduce another part of r249367. This commit fixes a register leak in
dt_cg_ptrsize() and generally cleans up some of the error handling around
register allocation.

This change corresponds to part of illumos-gate commit e5803b76927480:
3025 register leak in D code generation

Reviewed by: pfg
Obtained from: illumos
MFC after: 1 month

Update vendor/illumos to illumos-gate 13758:23432da34147 (dtrace changes)

References:
https://www.illumos.org/issues/3021
https://www.illumos.org/issues/3022
https://www.illumos.org/issues/3023
https://www.illumos.org/issues/3024
https://www.illumos.org/issues/3025
https://www.illumos.org/issues/3026

Obtained from: ssh://anonhg@hg.illumos.org/illumos-gate