rs6000: Support variable insert and Expand vec_insert in expander [PR79251]

rs6000: Support variable insert and Expand vec_insert in expander [PR79251]

vec_insert accepts 3 arguments, arg0 is input vector, arg1 is the value
to be insert, arg2 is the place to insert arg1 to arg0.  Current expander
generates stxv+stwx+lxv if arg2 is variable instead of constant, which
causes serious store hit load performance issue on Power.  This patch tries
 1) Build VIEW_CONVERT_EXPR for vec_insert (i, v, n) like v[n&3] = i to
unify the gimple code, then expander could use vec_set_optab to expand.
 2) Expand the IFN VEC_SET to fast instructions: lvsr+insert+lvsl.
In this way, "vec_insert (i, v, n)" and "v[n&3] = i" won't be expanded too
early in gimple stage if arg2 is variable, avoid generating store hit load
instructions.

For Power9 V4SI:
	addi 9,1,-16
	rldic 6,6,2,60
	stxv 34,-16(1)
	stwx 5,9,6
	lxv 34,-16(1)
=>
	rlwinm 6,6,2,28,29
	mtvsrwz 0,5
	lvsr 1,0,6
	lvsl 0,0,6
	xxperm 34,34,33
	xxinsertw 34,0,12
	xxperm 34,34,32

Though instructions increase from 5 to 7, the performance is improved
60% in typical cases.
Tested with V2DI, V2DF V4SI, V4SF, V8HI, V16QI on Power9-LE.

2021-01-22  Xionghu Luo  <luoxhu@linux.ibm.com>

gcc/ChangeLog:

	PR target/79251
	PR target/98065

	* config/rs6000/rs6000-c.c (altivec_resolve_overloaded_builtin):
	Ajdust variable index vec_insert from address dereference to
	ARRAY_REF(VIEW_CONVERT_EXPR) tree expression.
	* config/rs6000/rs6000-protos.h (rs6000_expand_vector_set_var):
	New declaration.
	* config/rs6000/rs6000.c (rs6000_expand_vector_set_var): New function.

2021-01-22  Xionghu Luo  <luoxhu@linux.ibm.com>

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr79251.p9.c: New test.
	* gcc.target/powerpc/pr79251-run.c: New test.
	* gcc.target/powerpc/pr79251.h: New header.