cmd/internal/gc: mark panicindex calls as not returning

cmd/internal/gc: mark panicindex calls as not returning

Most of the calls to panicindex are already
marked as not returning, but these two were missed
at some point.

Performance changes below.

name                   old mean              new mean              delta
BinaryTree17            5.70s × (0.98,1.04)   5.68s × (0.97,1.04)    ~    (p=0.681)
Fannkuch11              4.32s × (1.00,1.00)   4.41s × (0.98,1.03)  +1.98% (p=0.018)
FmtFprintfEmpty        92.6ns × (0.91,1.11)  92.7ns × (0.91,1.16)    ~    (p=0.969)
FmtFprintfString        280ns × (0.97,1.05)   281ns × (0.96,1.08)    ~    (p=0.860)
FmtFprintfInt           284ns × (0.99,1.02)   288ns × (0.97,1.06)    ~    (p=0.207)
FmtFprintfIntInt        488ns × (0.98,1.01)   493ns × (0.97,1.04)    ~    (p=0.271)
FmtFprintfPrefixedInt   418ns × (0.98,1.04)   423ns × (0.97,1.04)    ~    (p=0.311)
FmtFprintfFloat         597ns × (1.00,1.00)   598ns × (0.99,1.01)    ~    (p=0.789)
FmtManyArgs            1.87µs × (0.99,1.01)  1.89µs × (0.98,1.05)    ~    (p=0.158)
GobDecode              14.6ms × (0.99,1.01)  14.8ms × (0.98,1.03)  +1.51% (p=0.015)
GobEncode              12.3ms × (0.98,1.03)  12.3ms × (0.98,1.01)    ~    (p=0.474)
Gzip                    647ms × (1.00,1.01)   656ms × (0.99,1.05)    ~    (p=0.104)
Gunzip                  142ms × (1.00,1.00)   142ms × (1.00,1.00)    ~    (p=0.110)
HTTPClientServer       89.6µs × (0.99,1.03)  91.2µs × (0.97,1.04)    ~    (p=0.061)
JSONEncode             31.7ms × (0.99,1.01)  32.6ms × (0.97,1.08)  +2.87% (p=0.038)
JSONDecode              111ms × (1.00,1.01)   114ms × (0.97,1.05)  +2.47% (p=0.040)
Mandelbrot200          6.01ms × (1.00,1.00)  6.11ms × (0.98,1.04)    ~    (p=0.073)
GoParse                6.54ms × (0.99,1.02)  6.66ms × (0.97,1.04)    ~    (p=0.064)
RegexpMatchEasy0_32     159ns × (0.99,1.02)   159ns × (0.99,1.00)    ~    (p=0.693)
RegexpMatchEasy0_1K     540ns × (0.99,1.03)   538ns × (1.00,1.01)    ~    (p=0.360)
RegexpMatchEasy1_32     137ns × (0.99,1.01)   138ns × (1.00,1.00)    ~    (p=0.511)
RegexpMatchEasy1_1K     867ns × (1.00,1.01)   869ns × (0.99,1.01)    ~    (p=0.193)
RegexpMatchMedium_32    252ns × (1.00,1.00)   252ns × (0.99,1.01)    ~    (p=0.076)
RegexpMatchMedium_1K   72.7µs × (1.00,1.00)  72.7µs × (1.00,1.00)    ~    (p=0.963)
RegexpMatchHard_32     3.84µs × (1.00,1.00)  3.85µs × (1.00,1.00)    ~    (p=0.371)
RegexpMatchHard_1K      117µs × (1.00,1.01)   118µs × (1.00,1.00)    ~    (p=0.898)
Revcomp                 909ms × (0.98,1.03)   920ms × (0.97,1.07)    ~    (p=0.368)
Template                128ms × (0.99,1.01)   129ms × (0.98,1.03)  +1.41% (p=0.042)
TimeParse               619ns × (0.98,1.01)   619ns × (0.99,1.01)    ~    (p=0.730)
TimeFormat              651ns × (1.00,1.01)   661ns × (0.98,1.04)    ~    (p=0.097)

Change-Id: I0ec5baff41f5d282307137ce0d927e6301e4fa10
Reviewed-on: https://go-review.googlesource.com/9811
Reviewed-by: David Chase <drchase@google.com>