The "*neon_mov<mode>" patterns for 128 bit sized quantities uses the "Dn"
constraint to match vmov.f32 and vmov.i<vec-width> patterns.
This constraint boils down to using the `neon_immediate_valid` function.
Once the constraint has matched, the output C statement asserts that function
passes.

The output C statement calls `neon_immediate_valid` with the mode taken from the
iterator, while the constraint takes the mode from the operand.
This can cause a discrepency when the operand is a CONST_INT, as the constraint
passes VOIDmode which `neon_immediate_valid` treats as DImode, while the C
statement passes the mode of the iterator which can be TImode.
When this happens, the `neon_immediate_valid` can fail in the second call (if
e.g. the CONST_INT is a valid immediate in DImode but not TImode) which would
trigger the assertion.

The testcase added with this patch triggers this when compiled with an arm cross
compiler using the command line below.
gcc -march=armv8-a -c neon-immediate-timode.c -O1 -mfloat-abi=hard -mfpu=neon-fp-armv8

This patch splits the original "Dn" constraint into three new constraints, "DN"
for TImode CONST_INT, "Dn" for DImode CONST_INT, and "Dm" for CONST_VECTOR.
Splitting things up this way requires using one extra alternative in the
"*neon_mov<mode>" patterns, but makes it clear from the constraint what mode is
being used.

We also remove the behaviour of treating VOIDmode as DImode in
`neon_valid_immediate` since the original "Dn" constraint was the only place
that functionality was used.  VOIDmode is now never passed to that function.
An assertion has been added to the function to ensure this problem is caught
earlier on.

Bootstrapped on arm-none-linux-gnueabihf
Regtested on cross-compiler arm-none-eabi

gcc/ChangeLog:

2019-04-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	PR target/90024
	* config/arm/arm.c (neon_valid_immediate): Disallow VOIDmode parameter.
	* config/arm/constraints.md (Dm, DN, Dn): Split previous Dn constraint
	into three.
	* config/arm/neon.md (*neon_mov<mode>): Account for TImode and DImode
	differences directly.
	(*smax<mode>3_neon, vashl<mode>3, vashr<mode>3_imm): Use Dm constraint.

gcc/testsuite/ChangeLog:

2019-04-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	PR target/90024
	* gcc.dg/torture/neon-immediate-timode.c: New test.

From-SVN: r270226