MdeModulePkg/Tpl: Fix negative value left shift
MdeModulePkg/Tpl: Fix negative value left shift

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=695

Within function CoreRestoreTpl(), left shift a negative value -2 is used
in:
"while (((-2 << NewTpl) & gEventPending) != 0) {"

which involves undefined behavior.

According to the C11 spec, Section 6.5.7:
> 4 The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
>   bits are filled with zeros. If E1 has an unsigned type, the value
>   of the result is E1 * 2^E2 , reduced modulo one more than the
>   maximum value representable in the result type. If E1 has a signed
>   type and nonnegative value, and E1 * 2^E2 is representable in the
>   result type, then that is the resulting value; otherwise, the
>   behavior is undefined.

This commit refines the code logic to avoid left shifting the negative
value.

Cc: Steven Shi <4f7a0884debfd9139745b184e69c58dae2b3e6ce@intel.com>
Cc: Eric Dong <fa9eeec52367040ac98013bc4b2913db170588f8@intel.com>
Cc: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>
Cc: Michael Kinney <fd20bc543a9f65bb633fac7d08a403c1f1c8eb6c@intel.com>
Cc: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <f387b71200577e27ccf5abb090fda0ddd187ec6e@intel.com>
Reviewed-by: Star Zeng <5c6bb72efe464613af94e84c05522a361b23844a@intel.com>

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