[Committed] PR middle-end/102031: Fix typo/mistake in simplify_truncation patch.

[Committed] PR middle-end/102031: Fix typo/mistake in simplify_truncation patch.

My apologies again.  My patch to simplify truncations of SUBREGs in
simplify-rtx.c contained an error where I'd accidentally compared
against a mode instead of the precision of that mode.  Grr!  It even
survived regression testing on two platforms.  Fixed below, and
committed as obvious, after a full "make bootstrap" and "make -k check"
on x86_64-pc-linux-gnu with no new regressions.

2021-08-24  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR middle-end/102031
	* simplify-rtx.c (simplify_truncation): When comparing precisions
	use "subreg_prec" variable, not "subreg_mode".