i386: Narrow test instructions with immediate operands [PR111698]

Narrow test instructions with immediate operand that test memory location
for zero.  E.g. testl $0x00aa0000, mem can be converted to testb $0xaa, mem+2.
Reject targets where reading (possibly unaligned) part of memory location
after a large write to the same address causes store-to-load forwarding stall.

    PR target/111698

gcc/ChangeLog:

    * config/i386/x86-tune.def (X86_TUNE_PARTIAL_MEMORY_READ_STALL):
    New tune.
    * config/i386/i386.h (TARGET_PARTIAL_MEMORY_READ_STALL): New macro.
    * config/i386/i386.md: New peephole pattern to narrow test
    instructions with immediate operands that test memory locations
    for zero.

gcc/testsuite/ChangeLog:

    * gcc.target/i386/pr111698.c: New test.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Uros.