The following piglit tests fail on all Intel hardware, with -O0:

    piglit.spec.ext_texture_compression_s3tc.fbo-generatemipmap-formats
    piglit.spec.ext_texture_srgb.fbo-generatemipmap-formats-s3tc

EG:
piglit/bin/fbo-generatemipmap-formats GL_EXT_texture_compression_s3tc -auto -fbo
Using test set: GL_EXT_texture_compression_s3tc
Testing GL_COMPRESSED_RGB_S3TC_DXT1_EXT
PIGLIT: {"subtest": {"GL_COMPRESSED_RGB_S3TC_DXT1_EXT" : "pass"}}
Testing GL_COMPRESSED_RGBA_S3TC_DXT1_EXT
PIGLIT: {"subtest": {"GL_COMPRESSED_RGBA_S3TC_DXT1_EXT" : "pass"}}
Testing GL_COMPRESSED_RGBA_S3TC_DXT3_EXT
PIGLIT: {"subtest": {"GL_COMPRESSED_RGBA_S3TC_DXT3_EXT" : "pass"}}
Testing GL_COMPRESSED_RGBA_S3TC_DXT5_EXT
PIGLIT: {"subtest": {"GL_COMPRESSED_RGBA_S3TC_DXT5_EXT" : "pass"}}
Testing GL_COMPRESSED_RGB_S3TC_DXT1_EXT (NPOT)
PIGLIT: {"subtest": {"GL_COMPRESSED_RGB_S3TC_DXT1_EXT NPOT" : "pass"}}
Testing GL_COMPRESSED_RGBA_S3TC_DXT1_EXT (NPOT)
PIGLIT: {"subtest": {"GL_COMPRESSED_RGBA_S3TC_DXT1_EXT NPOT" : "pass"}}
Testing GL_COMPRESSED_RGBA_S3TC_DXT3_EXT (NPOT)
PIGLIT: {"subtest": {"GL_COMPRESSED_RGBA_S3TC_DXT3_EXT NPOT" : "pass"}}
Testing GL_COMPRESSED_RGBA_S3TC_DXT5_EXT (NPOT)
Probe color at (439,4)
  Expected: 0 255 0 63
  Observed: 0 255 0 255
PIGLIT: {"subtest": {"GL_COMPRESSED_RGBA_S3TC_DXT5_EXT NPOT" : "fail"}}
PIGLIT: {"result": "fail" }

When Mesa is compiled with -O2, the tests pass.

You would expect optimizations to *break* tests, not fix them.  ¯\_(ツ)_/¯