target/ppc: Fix FPSCR.FI changing in float_overflow_excp()

target/ppc: Fix FPSCR.FI changing in float_overflow_excp()

This patch fixes another not-so-clear situation in Power ISA
regarding the inexact bits in FPSCR. The ISA states that:

"""
When Overflow Exception is disabled (OE=0) and an
Overflow Exception occurs, the following actions are
taken:
...
2. Inexact Exception is set
XX <- 1
...
FI is set to 1
...
"""

However, when tested on a Power 9 hardware, some instructions that
trigger an OX don't set the FI bit:

xvcvdpsp(0x4050533fcdb7b95ff8d561c40bf90996) = FI: CLEARED -> CLEARED
xvnmsubmsp(0xf3c0c1fc8f3230, 0xbeaab9c5) = FI: CLEARED -> CLEARED
(just a few examples. Other instructions are also affected)

The root cause for this seems to be that only instructions that list
the bit FI in the "Special Registers Altered" should modify it.

QEMU is, today, not working like the hardware:

xvcvdpsp(0x4050533fcdb7b95ff8d561c40bf90996) = FI: CLEARED -> SET
xvnmsubmsp(0xf3c0c1fc8f3230, 0xbeaab9c5) = FI: CLEARED -> SET

(all tests assume FI is cleared beforehand)

Fix this by making float_overflow_excp() return float_flag_inexact
if it should update the inexact flags.

Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Rashmica Gupta <rashmica.g@gmail.com>
Message-Id: <20220517161522.36132-3-victor.colombo@eldorado.org.br>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>