Revert g-465c7c89e92a6d6d582173e505cb16dcb9873034

Revert g-465c7c89e92a6d6d582173e505cb16dcb9873034

The patch caused regressions in gcc.target/sh/pr64345-1.c on
sh3-linux-gnu and gcc.target/m68k/pr39726.c on m68k-linux-gnu.
It didn't look like they would be fixable in an acceptably
non-invasive and unhacky way, so punting till future releases.

2020-01-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	Revert:

	2020-01-28  Richard Sandiford  <richard.sandiford@arm.com>

	PR rtl-optimization/87763
	* simplify-rtx.c (simplify_truncation): Extend sign/zero_extract
	simplification to handle subregs as well as bare regs.
	* config/i386/i386.md (*testqi_ext_3): Match QI extracts too.