combine: Don't mess with subregs of floating point (PR78590)

combine: Don't mess with subregs of floating point (PR78590)

PR78590 shows a problem in change_zero_ext, where we change a zero_extend
of a subreg to a logical and.  We should only do this if the thing we are
taking the subreg of is a scalar integer, otherwise we will take a subreg
of (e.g.) a float in a different size, which is nonsensical and hits an
assert.

	PR rtl-optimization/78590
	* combine.c (change_zero_ext): Transform zero_extend of subregs only
	if the subreg_reg is a scalar integer mode.

From-SVN: r242999