stor-layout: Create DECL_BIT_FIELD_REPRESENTATIVE even for bitfields in unions [PR101062]

stor-layout: Create DECL_BIT_FIELD_REPRESENTATIVE even for bitfields in unions [PR101062]

The following testcase is miscompiled on x86_64-linux, the bitfield store
is implemented as a RMW 64-bit operation at d+24 when the d variable has
size of only 28 bytes and scheduling moves in between the R and W part
a store to a different variable that happens to be right after the d
variable.

The reason for this is that we weren't creating
DECL_BIT_FIELD_REPRESENTATIVEs for bitfields in unions.

The following patch does create them, but treats all such bitfields as if
they were in a structure where the particular bitfield is the only field.

2021-06-16  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101062
	* stor-layout.c (finish_bitfield_representative): For fields in unions
	assume nextf is always NULL.
	(finish_bitfield_layout): Compute bit field representatives also in
	unions, but handle it as if each bitfield was the only field in the
	aggregate.

	* gcc.dg/pr101062.c: New test.