c-family: Tighten vector handling in type_for_mode [PR94072]

c-family: Tighten vector handling in type_for_mode [PR94072]

In this PR we had a 512-bit VECTOR_TYPE whose mode is XImode
(an integer mode used for four 128-bit vectors).  When trying
to expand a zero constant for it, we hit code in expand_expr_real_1
that tries to use the associated integer type instead.  The code used
type_for_mode (XImode, 1) to get this integer type.

However, the c-family implementation of type_for_mode checks for
any registered built-in type that matches the mode and has the
right signedness.  This meant that it could return a built-in
vector type when given an integer mode (particularly if, as here,
the vector type isn't supported by the current subtarget and so
TYPE_MODE != TYPE_MODE_RAW).  The expand code would then cycle
endlessly trying to use this "new" type instead of the original
vector type.

2020-03-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/c-family/
	PR middle-end/94072
	* c-common.c (c_common_type_for_mode): Before using a registered
	built-in type, check that the vectorness of the type matches
	the vectorness of the mode.

gcc/testsuite/
	PR middle-end/94072
	* gcc.target/aarch64/pr94072.c: New test.