This is a regression between these two versions:

gcc version 10.0.1 20200324 (experimental) (GCC) --- OK
gcc version 10.0.1 20200402 (experimental) (GCC) --- 25839b6af9f10b3227719db35b9e9e38320341e7 fails

It is likely the version that passed was b599bf9d6d1e180d350b71e51e08a66a1bb1546a but I am not 100% certain.

How to reproduce:

$ git clone https://github.com/glennrp/libpng
$ cd libpng
$ git checkout v1.6.37
$ CC=/opt/gcc10-for-tng/bin/gcc-10 CXX=/opt/gcc10-for-tng/bin/g++-10 ./configure
$ make
$ /opt/gcc10-for-tng/bin/gcc-10 -DPNG_INTEL_SSE_OPT=1 -I. -O3 -DNDEBUG -o pngrutil.opt.o -c pngrutil.c
pngrutil.c: In function 'png_combine_row':
pngrutil.c:3200:1: error: missing definition
 3200 | png_combine_row(png_const_structrp png_ptr, png_bytep dp, int display)
      | ^~~~~~~~~~~~~~~
for SSA_NAME: _350 in statement:
dp32_155 = _350;
during GIMPLE pass: vect
pngrutil.c:3200:1: internal compiler error: verify_ssa failed
0xff15f2 verify_ssa(bool, bool)
        ../../gcc/gcc/tree-ssa.c:1208
0xd0abb5 execute_function_todo
        ../../gcc/gcc/passes.c:1992
0xd0b8fc do_per_function
        ../../gcc/gcc/passes.c:1640
0xd0b8fc execute_todo
        ../../gcc/gcc/passes.c:2039
Please submit a full bug report,
with preprocessed source if appropriate.