[Bug target/79799] New: Improve vec_insert of float on Power9

["meissner at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79799

            Bug ID: 79799
           Summary: Improve vec_insert of float on Power9
           Product: gcc
           Version: 7.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: meissner at gcc dot gnu.org
  Target Milestone: ---

At the moment, we do not have special code on power9 to have vec_insert of
single precision floating point.

So for:

vector float insert_0 (vector float vf, float f) { return vec_insert (f, vf,
0); }

we currently generate:

insert_0:
.LCF2:
0:      addis 2,12,.TOC.-.LCF2@ha
        addi 2,2,.TOC.-.LCF2@l
        .localentry     insert_0,.-insert_0
        addis 9,2,.LC0@toc@ha
        addi 10,1,-16
        stfs 1,-16(1)
        addi 9,9,.LC0@toc@l
        lvewx 0,0,10
        lxv 33,0(9)
        vpermr 2,0,2,1
        blr
        .section        .rodata.cst16,"aM",@progbits,16
        .align 4
.LC0:
        .byte   16
        .byte   17
        .byte   18
        .byte   19
        .byte   4
        .byte   5
        .byte   6
        .byte   7
        .byte   8
        .byte   9
        .byte   10
        .byte   11
        .byte   12
        .byte   13
        .byte   14
        .byte   15

Instead we should generate a convert to XSCVDPSPN, right shift 32-bits, and
XXINSERTW.