middle-end: Correct calculation of mul_widen_cost and mul_highpart_cost.

This patch fixes a subtle bug in the depths of GCC's synth_mult,
where the middle-end queries whether (how well) the target supports
widening and highpart multiplications by calling targetm.rtx_costs.
The code in init_expmed and init_expmed_one_mode iterates over various
RTL patterns querying the cost of each.  To avoid generating & garbage
collecting too much junk, it reuses the same RTL over and over, but
adjusting the modes between each call.

Alas this reuse of state is a little fragile, and at some point a
change to init_expmed_one_conv has resulted in the state (mode of
a register) being changed, but not reset before being used again.

Using the old software engineering/defensive programming maxim of
"why fix a bug just once, if it can be fixed in multiple places",
this patch both restores the original value in init_expmed_one_conv,
and also sets it to the expected value in init_expmed_one_mode.
This should hopefully signal the need to be careful of invariants for
anyone modifying this code in future.

2020-08-09  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* expmed.c (init_expmed_one_conv): Restore all->reg's mode.
	(init_expmed_one_mode): Set all->reg to desired mode.

gcc/testsuite/ChangeLog
	PR target/71321
	* gcc.target/i386/pr71321.c: Check that the code doesn't use
	the 4B zero displacement lea, not that it uses lea.