arm: Auto-vectorization for MVE: vshl

This patch enables MVE vshlq instructions for auto-vectorization.

The existing mve_vshlq_n_<supf><mode> is kept, as it takes a single
immediate as second operand, and is used by arm_mve.h.

We move the vashl<mode>3 insn from neon.md to an expander in
vec-common.md, and the mve_vshlq_<supf><mode> insn from mve.md to
vec-common.md, adding the second alternative fron neon.md.

mve_vshlq_<supf><mode> will be used by a later patch enabling
vectorization for vshr, as a unified version of
ashl3<mode3>_[signed|unsigned] from neon.md. Keeping the use of unspec
VSHLQ enables to generate both 's' and 'u' variants.

It is not clear whether the neon_shift_[reg|imm]<q> attribute is still
suitable, since this insn is also used for MVE.

I kept the mve_vshlq_<supf><mode> naming instead of renaming it to
ashl3_<supf>_<mode> as discussed because the reference in
arm_mve_builtins.def automatically inserts the "mve_" prefix and I
didn't want to make a special case for this.

I haven't yet found why the v16qi and v8hi tests are not vectorized.
With dest[i] = a[i] << b[i] and:
  {
    int i;
    unsigned int i.24_1;
    unsigned int _2;
    int16_t * _3;
    short int _4;
    int _5;
    int16_t * _6;
    short int _7;
    int _8;
    int _9;
    int16_t * _10;
    short int _11;
    unsigned int ivtmp_42;
    unsigned int ivtmp_43;

    <bb 2> [local count: 119292720]:

    <bb 3> [local count: 954449105]:
    i.24_1 = (unsigned int) i_23;
    _2 = i.24_1 * 2;
    _3 = a_15(D) + _2;
    _4 = *_3;
    _5 = (int) _4;
    _6 = b_16(D) + _2;
    _7 = *_6;
    _8 = (int) _7;
    _9 = _5 << _8;
    _10 = dest_17(D) + _2;
    _11 = (short int) _9;
    *_10 = _11;
    i_19 = i_23 + 1;
    ivtmp_42 = ivtmp_43 - 1;
    if (ivtmp_42 != 0)
      goto <bb 5>; [87.50%]
    else
      goto <bb 4>; [12.50%]

    <bb 5> [local count: 835156386]:
    goto <bb 3>; [100.00%]

    <bb 4> [local count: 119292720]:
    return;

  }
the vectorizer says:
mve-vshl.c:37:96: note:   ==> examining statement: _5 = (int) _4;
mve-vshl.c:37:96: note:   vect_is_simple_use: operand *_3, type of def: internal
mve-vshl.c:37:96: note:   vect_is_simple_use: vectype vector(8) short int
mve-vshl.c:37:96: missed:   conversion not supported by target.
mve-vshl.c:37:96: note:   vect_is_simple_use: operand *_3, type of def: internal
mve-vshl.c:37:96: note:   vect_is_simple_use: vectype vector(8) short int
mve-vshl.c:37:96: note:   vect_is_simple_use: operand *_3, type of def: internal
mve-vshl.c:37:96: note:   vect_is_simple_use: vectype vector(8) short int
mve-vshl.c:37:117: missed:   not vectorized: relevant stmt not supported: _5 = (int) _4;
mve-vshl.c:37:96: missed:  bad operation or unsupported loop bound.
mve-vshl.c:37:96: note:  ***** Analysis failed with vector mode V8HI

2020-12-03  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/mve.md (mve_vshlq_<supf><mode>): Move to
	vec-commond.md.
	* config/arm/neon.md (vashl<mode>3): Delete.
	* config/arm/vec-common.md (mve_vshlq_<supf><mode>): New.
	(vasl<mode>3): New expander.

	gcc/testsuite/
	* gcc.target/arm/simd/mve-vshl.c: Add tests for vshl.