perf expr: Allow a double if expression

Some TMA metrics have double if expressions like:

  ( CPU_CLK_UNHALTED.THREAD / 2 ) * ( 1 + CPU_CLK_UNHALTED.ONE_THREAD_ACTIVE / CPU_CLK_UNHALTED.REF_XCLK ) ) if #core_wide < 1 else ( CPU_CLK_UNHALTED.THREAD_ANY / 2 ) if #SMT_on else CPU_CLK_UNHALTED.THREAD

This currently fails to parse as the left hand side if expression needs
to be in parentheses. By allowing the if expression to have a right hand
side that is an if expression we can parse the expression above, with
left to right evaluation order that matches languages like Python.

Signed-off-by: Ian Rogers <[email protected]>
Cc: Ahmad Yasin <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Caleb Biggers <[email protected]>
Cc: Florian Fischer <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: John Garry <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Kshipra Bopardikar <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Miaoqian Lin <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Perry Taylor <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Samantha Alt <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Richter <[email protected]>
Cc: Xing Zhengjun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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