middle-end/105715 - missed RTL if-conversion with COND_EXPR expansion

middle-end/105715 - missed RTL if-conversion with COND_EXPR expansion

When the COND_EXPR condition operand was split out to a separate stmt
it became subject to CSE with other condition evaluations.  This
unfortunately leads to TER no longer applying and in turn RTL
expansion of COND_EXPRs no longer seeing the condition and thus
failing to try conditional move expansion.  This can be seen with
gcc.target/i386/pr45685.c when built with -march=cascadelake which
then FAILs to produce the expected number of cmovs.

It can also be seen when we create more COND_EXPRs early like for
instruction selection of MIN/MAX operations that map to IEEE
a > b ? a : b expression semantics.

	PR middle-end/105715
	* gimple-isel.cc (gimple_expand_vec_exprs): Merge into...
	(pass_gimple_isel::execute): ... this.  Duplicate
	comparison defs of COND_EXPRs.