sra: Fix sra_modify_expr handling of partial writes (PR 94482)

sra: Fix sra_modify_expr handling of partial writes (PR 94482)

when sra_modify_expr is invoked on an expression that modifies only
part of the underlying replacement, such as a BIT_FIELD_REF on a LHS
of an assignment and the SRA replacement's type is not compatible with
what is being replaced (0th operand of the B_F_R in the above
example), it does not work properly, basically throwing away the partd
of the expr that should have stayed intact.

This is fixed in two ways.  For BIT_FIELD_REFs, which operate on the
binary image of the replacement (and so in a way serve as a
VIEW_CONVERT_EXPR) we just do not bother with convertsing.  For
REALPART_EXPRs and IMAGPART_EXPRs, if the replacement is not a
register, we insert a VIEW_CONVERT_EXPR under
the complex partial access expression, which is always OK, for loads
from registers we take the extra step of converting it to a temporary.

This revealed a bug in fwprop which is fixed with the hunk from Richi.

The testcase for handling REALPART_EXPR and IMAGPART_EXPR is a bit
fragile because SRA prefers complex and vector types over anything
else (and in between the two it decides based on TYPE_UID which in my
testing today always preferred complex types) and so I only run it at
-O1 (which is the only level where the the test fails for me).

Bootstrapped and tested on x86_64-linux, i686-linux and aarch64-linux.

2020-04-09  Martin Jambor  <mjambor@suse.cz>
	    Richard Biener  <rguenther@suse.de>

	PR tree-optimization/94482
	* tree-sra.c (create_access_replacement): Dump new replacement with
	TDF_UID.
	(sra_modify_expr): Fix handling of cases when the original EXPR writes
	to only part of the replacement.
	* tree-ssa-forwprop.c (pass_forwprop::execute): Properly verify
	the first operand of combinations into REAL/IMAGPART_EXPR and
	BIT_FIELD_REF.

	testsuite/
	* gcc.dg/torture/pr94482.c: New test.
	* gcc.dg/tree-ssa/pr94482-2.c: Likewise.